Add expressive diagnostics for Warning::UnusedReturnValue

[ironcev]

Description

This PR adds expressive diagnostics for the UnusedReturnValue warning that explains how to intentionally ignore a returned value by using let _ = ....

The reason for adding the diagnostics was an observed confusion among novice Sway developers in the particular case of using __dbg intrinsic as a statement:

__dbg("some debug message");

It was unclear to developers why this usage generates the warning, and also how to remove it.

Additionally, the PR fixes some uninlined_format_args Clippy warnings.

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 #7382 will not alter performance

_{Comparing ironcev/expressive-diagnostics-for-unused-return-value (828ec9d) with master (608fc52)}

Summary

✅ 25 untouched benchmarks