Add build to .gitignore in sui move new (MystenLabs#20395)

## Description 

- Adds `build/*` to `.gitignore` when invoking `sui move new` (extending
it if it already exists)
 - Splits out `move new` implementation into smaller functions

## Test plan 

I added to the `move new` unit tests; checking the case where the
`.gitignore` already exists, and extended the existing tests to check
that it was added.

Note that I didn't add tests to `sui move new`, only `move new`.

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [x] CLI: When issuing `sui move new` command, a `.gitignore` file will
be added to the project's folder.
- [ ] Rust SDK:
- [ ] REST API:

external-crates/move/crates/move-cli/src/base/new.rs

@@ -1,18 +1,13 @@
 // Copyright (c) The Move Contributors
 // SPDX-License-Identifier: Apache-2.0
 
-use anyhow::anyhow;
+use anyhow::{self, ensure, Context};
 use clap::*;
 use move_core_types::identifier::Identifier;
 use move_package::source_package::layout::SourcePackageLayout;
+use std::io::{BufRead, BufReader};
 use std::{fmt::Display, fs::create_dir_all, io::Write, path::Path};
 
-// TODO get a stable path to this stdlib
-// pub const MOVE_STDLIB_PACKAGE_NAME: &str = "MoveStdlib";
-// pub const MOVE_STDLIB_PACKAGE_PATH: &str = "{ \
-//     git = \"https://github.com/move-language/move.git\", \
-//     subdir = \"language/move-stdlib\", rev = \"main\" \
-// }";
 pub const MOVE_STDLIB_ADDR_NAME: &str = "std";
 pub const MOVE_STDLIB_ADDR_VALUE: &str = "0x1";
 
@@ -43,17 +38,53 @@ impl New {
         custom: &str, // anything else that needs to end up being in Move.toml (or empty string)
     ) -> anyhow::Result<()> {
         // TODO warn on build config flags
-        let Self { name } = self;
 
-        if !Identifier::is_valid(&name) {
-            return Err(anyhow!(
-                "Invalid package name. Package name must start with a lowercase letter \
-                 and consist only of lowercase letters, numbers, and underscores."
-            ));
-        }
+        ensure!(
+            Identifier::is_valid(&self.name),
+            "Invalid package name. Package name must start with a lowercase letter \
+                     and consist only of lowercase letters, numbers, and underscores."
+        );
 
-        let path = path.unwrap_or_else(|| Path::new(&name));
+        let path = path.unwrap_or_else(|| Path::new(&self.name));
         create_dir_all(path.join(SourcePackageLayout::Sources.path()))?;
+
+        self.write_move_toml(path, deps, addrs, custom)?;
+        self.write_gitignore(path)?;
+        Ok(())
+    }
+
+    /// add `build/*` to `{path}/.gitignore` if it doesn't already have it
+    fn write_gitignore(&self, path: &Path) -> anyhow::Result<()> {
+        let gitignore_entry = "build/*";
+
+        let mut file = std::fs::OpenOptions::new()
+            .create(true)
+            .truncate(false)
+            .read(true)
+            .write(true)
+            .open(path.join(".gitignore"))
+            .context("Unexpected error creating .gitignore")?;
+
+        for line in BufReader::new(&file).lines().map_while(Result::ok) {
+            if line == gitignore_entry {
+                return Ok(());
+            }
+        }
+
+        writeln!(file, "{gitignore_entry}")?;
+        Ok(())
+    }
+
+    /// create default `Move.toml`
+    fn write_move_toml(
+        &self,
+        path: &Path,
+        deps: impl IntoIterator<Item = (impl Display, impl Display)>,
+        addrs: impl IntoIterator<Item = (impl Display, impl Display)>,
+        custom: &str, // anything else that needs to end up being in Move.toml (or empty string)
+    ) -> anyhow::Result<()> {
+        let Self { name } = self;
+
         let mut w = std::fs::File::create(path.join(SourcePackageLayout::Manifest.path()))?;
         writeln!(
             w,

external-crates/move/crates/move-cli/tests/build_tests/new_clobber/gitignore_exists/args.exp

@@ -0,0 +1,8 @@
+Command `new -p package example`:
+External Command `cat package/.gitignore`:
+existing_ignore
+build/*
+External Command `ls -A package`:
+.gitignore
+Move.toml
+sources

external-crates/move/crates/move-cli/tests/build_tests/new_clobber/gitignore_exists/args.txt

@@ -0,0 +1,4 @@
+# if `.gitignore` exists, everything should be created and .gitignore should be extended
+new -p package example
+>cat package/.gitignore
+>ls -A package

external-crates/move/crates/move-cli/tests/build_tests/new_clobber/gitignore_exists/package/.gitignore

@@ -0,0 +1 @@
+existing_ignore

external-crates/move/crates/move-cli/tests/build_tests/simple_new/args.exp → external-crates/move/crates/move-cli/tests/build_tests/new_simple/args.exp

@@ -35,6 +35,8 @@ edition = "2024.beta" # edition = "legacy" to use legacy (pre-2024) Move
 # and `--dev` modes.
 # alice = "0xB0B"
 
+External Command `cat P1/.gitignore`:
+build/*
 Command `new P2 -p other_dir`:
 External Command `cat other_dir/Move.toml`:
 [package]
@@ -72,3 +74,5 @@ edition = "2024.beta" # edition = "legacy" to use legacy (pre-2024) Move
 # and `--dev` modes.
 # alice = "0xB0B"
 
+External Command `cat other_dir/.gitignore`:
+build/*

external-crates/move/crates/move-cli/tests/build_tests/simple_new/args.txt → external-crates/move/crates/move-cli/tests/build_tests/new_simple/args.txt

@@ -1,5 +1,7 @@
 new P1
 > cat P1/Move.toml
+> cat P1/.gitignore
 
 new P2 -p other_dir
 > cat other_dir/Move.toml
+> cat other_dir/.gitignore