rust: add highlight scope for type parameters

[jmhain]

Highly generic code can be challenging to read when type parameters are highlighted the same as other types. This is especially true when the type parameters are not single letters, e.g. Fut is somewhat common in Rust.

So this PR adds a new highlighting scope for type parameters. I added it only for Rust as a proof-of-concept, but if accepted I'll update the queries for any other languages I'm familiar with (either in this PR or in a follow-up, whichever is preferred).

Here is a before and after with my in-progress port of the Codely theme.

[kirawi]

Looks good.

[kirawi]

There is also optional_type_parameter (which I didn't know was a thing until now) seen in

fn foo<T = Roo>() {
}

with an AST of

[source_file](https://tree-sitter.github.io/tree-sitter/playground#) [0, 0] - [2, 0]
  [function_item](https://tree-sitter.github.io/tree-sitter/playground#) [0, 0] - [1, 1]
    name: [identifier](https://tree-sitter.github.io/tree-sitter/playground#) [0, 3] - [0, 6]
    type_parameters: [type_parameters](https://tree-sitter.github.io/tree-sitter/playground#) [0, 6] - [0, 15]
      [optional_type_parameter](https://tree-sitter.github.io/tree-sitter/playground#) [0, 7] - [0, 14]
        name: [type_identifier](https://tree-sitter.github.io/tree-sitter/playground#) [0, 7] - [0, 8]
        default_type: [type_identifier](https://tree-sitter.github.io/tree-sitter/playground#) [0, 11] - [0, 14]
    parameters: [parameters](https://tree-sitter.github.io/tree-sitter/playground#) [0, 15] - [0, 17]
    body: [block](https://tree-sitter.github.io/tree-sitter/playground#) [0, 18] - [1, 1]

[jmhain]

Thanks for catching that @kirawi, I pushed a commit to handle that case

[cd-a]

Love it. Thanks!

As you mentioned, that would be awesome for other languages like TypeScript as well.

[jmhain]

@cd-a I was planning on doing TypeScript next 🙂

[the-mikedavis]

This makes sense to me 👍

[pascalkuthe]

I think this is generally a nice feature. Note that #1151 will cause issues here more frequently since generieren parameters often have larger scopes than function Parameters (impl Block in rust for example)