Fixes unreachable on invalid cfg attribute arg.

[esdrubal]

Description

We now throw an error that enforces cfg arg to be a valid one.

Fixes #6326

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 #6553 will degrade performances by 18.2%

_{Comparing esdrubal/unreachable_cfg (851b722) with master (ea4b9e9)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	esdrubal/unreachable_cfg	Change
❌	document_symbol	4.3 ms	5.3 ms	-18.2%

[tritao]

CodSpeed Performance Report

Merging #6553 will degrade performances by 18.2%

Comparing esdrubal/unreachable_cfg (851b722) with master (ea4b9e9)

Summary

❌ 1 regressions ✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

Benchmark master esdrubal/unreachable_cfg Change
❌ document_symbol 4.3 ms 5.3 ms -18.2%

I'm getting this same slowdown on one of my PRs.