const generics for standalone fns

[xunilrj]

Description

This PR is part of #6860 and allows standalone functions to have "const generics" like the code below:

fn return_n<const NNN: u64>() -> u64 {
    NNN
}

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 #7175 will not alter performance

_{Comparing xunilrj/standalone-fn-const-generics (b271777) with master (ee565f9)}

Summary

✅ 24 untouched benchmarks

[tritao]

Looks good, just a few nits