Add support for WebAssembly Interface Types

[eduardomourar]

Description

Add support for WebAssembly Interface Type (WIT):
https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on github.com.
    • Search results for each extension:
      • https://github.com/search?q=path%3A*.wit&type=code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/WebAssembly/wasi-io/tree/e6bcf18ac5e2d2deee5016fc4b6e260fc14d4bdc/wit
    • Sample license(s): Apache 2.0
  • I have included a syntax highlighting grammar: https://github.com/bytecodealliance/vscode-wit
  • I have added a color
    • Hex value: #6250e7
    • Rationale: Based on the WebAssembly logo

[Alhadis]

This is the first grammar I've encountered that uses repository fields in contexts that aren't the top-level grammar object. I see that VSCode supports this, but Atom doesn't—and by extension, that (probably) includes PrettyLights (GitHub's implementation of TextMate's highlighting system, which is closed-source).

So comments will end up looking something like this on GitHub:

This can be solved by moving every named rule to the top-level repository object.

[eduardomourar]

The repository fields have been moved to the top-level object.

[Alhadis]

Alright, grammar's looking good. The syntax highlighting grammar, I mean, not that of my Dunglish:

I noticed a few omissions in your grammar, but nothing urgent. I'll submit an issue to vscode-wit later with more info.

[eduardomourar]

@Alhadis, I believe the license needs to be reviewed. It should be straightforward because it is Apache 2.0 with LLVM exception.

[Alhadis]

/cc @lildude for license review

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.