Add type override rules

[Corecii]

This is an initial implementation of the settings described in this issue: #634 (comment)

It adds a setting to .project.json files to override types of files according to globs, like so:

{
  "name": "example",
  "typeOverrideRules": [
    {
      "pattern": "wally.lock",
      "use": "rojo/plaintext"
    },
    {
      "pattern": "**/*.md",
      "use": "rojo/plaintext"
    },
    {
      "pattern": "**/*.spec.lua",
      "use": "rojo/ignore"
    },
  ],
  "tree": {
    "$path": "src"
  }
}

The following types are implemented:

"rojo/plaintext"
"rojo/luau"
"rojo/luauserver"
"rojo/luauclient"
"rojo/json"
"rojo/csv"
"rojo/project"
"rojo/rbxm"
"rojo/rbxmx"
"rojo/jsonmodel"
"rojo/ignore"

...and anything else is captured as an Other(_) value, which currently bails if encountered since plugins aren't supported yet.

---

I'm not really sure if this is the way you'd want this implemented, but I took a crack at it because:

1. I'm interested in this feature!
2. Even if this implementation isn't used, the familiarity it builds for me with the Rojo codebase is valuable!
3. Maybe this implementation can be a starting point and improved upon? No worries if not.

[Corecii]

Some thoughts:

• do we want to call these "transformers"? I noticed that's the terminology you used, and I think it makes sense!
• is transformerRules a reasonable name for this project property?
• are we happy with refactoring snapshotting to be based on a Transformer type instead of deciding how to snapshot from the filename?
• are we happy with server, client, and module scripts being different types? These sync in as different objects so it makes sense. There are potential cases where you may need to specify something to sync in as a server script that isn't suffixed with .server.lua (or even .lua for that matter).

[Dekkonot]

Sorry about the 10 month delay on someone getting back to you on this!

I'm not sure on the term 'transformer' being used in project files because I associate it with modifying the file itself somehow. Since this is just using existing Rojo transformers, I think it's more fair to say we're just adding or changing file type associations. I'm not sure what a better name is though, since fileAssociations is probably a bad one.

I'm perfectly fine with changing snapshots to use transformer associations rather than the filename. Our current method is fine but I don't feel attached to it in lieu of this solution.

[Dekkonot]

As per our own discussions, I'm going to implement this myself.