Implements fully working hierarchical lexical scopes.

[esdrubal]

Description

This PR implements the missing elements for the hierarchical lexical scopes.
TypeCheckContext no longer uses cloned namespaces. Instead, we push and pop lexical scopes.
For this to work multiple places that used current_items now either use root_items or traverse the lexical scopes.

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 #6784 will degrade performances by 59.03%

_{Comparing esdrubal/hierarchical_lexical_scopes (23bcb58) with master (192f163)}

Summary

❌ 2 regressions
✅ 20 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	master	esdrubal/hierarchical_lexical_scopes	Change
❌	completion	21 ms	23.9 ms	-12.44%
❌	hover	1.8 ms	4.3 ms	-59.03%