- parse a let binding of type `let <IDENT> = <LITERAL>; in <IDENT>`
- can parse when multiple bindings in the let
- for now, concider every let as recursive let.

Have to translate nix let blocks to a nickel destructuring. Because
destructuring need a span, this commit make `FileId` required to parse
nix files. Also it was a good oportunity to add terms positions to
parsed terms.

patterns fields now.

tODO: update the pretty printer to print the full pattern an not only
the at binding. May be in a extra PR.

Make parsing of `record.${a field expr}` parsable.
It may not be parsed at term level but should not be an issue because it
effectively return a term and if not used at right place, the parser
will return an error anyway.

Co-authored-by: Yann Hamdaoui <[email protected]>

Add a filter to include `tests/nix/*.nix` files

in Nix `0.625` is not equivalant to `5/8`
even if it works in nickel, we prefer to stay correct and compare
frac with frac.

At the same time, it clean and simplify some parts of the code

Co-authored-by: Yann Hamdaoui <[email protected]>

Co-authored-by: Yann Hamdaoui <[email protected]>

- impl this trait for `rnix` operators and `SyntaxNode`
- others code cleaning

now able to evaluate `{...} ? <field_path>`

Identifiers require a position. So, we added it

- complex shadowing
- nested record combined with a with
- with and recursive record which does not pass for now  (check todos in
  `tests/nix/with.nix`)