Const generic feature toggle, parser and errors.

[xunilrj]

Description

This PR is part of #6860, and it is officially introducing the const_generic feature toggle.

For the moment, it is parsing syntax such as const N: u64 and it is returning an error explaining that the feature is off; on the other hand, it is also returning an error saying that const generics are still not supported in impl traits when the feature is on. Future PRs will replicate this error in all possible places.

Nothing else is implemented and it is reserved for future PRs.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6926 will not alter performance

_{Comparing xunilrj/const-generic-feature-toggle (0373c30) with master (1589951)}

Summary

✅ 22 untouched benchmarks

[tritao]

@FuelLabs/tooling can you review this one?