refactor/small improvements

[gnikit]

• refactor(ast): optimize scope filtering logic
• refactor(ast): update variable names for clarity
• refactor(ast): update variable typings for clarity
• refactor(ast): improve object lookup efficiency

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.36170% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 87.98%. Comparing base (9d582ba) to head (09e87a6).

Files	Patch %	Lines
fortls/parsers/internal/ast.py	88.37%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #388 +/- ## ========================================== + Coverage 87.73% 87.98% +0.25% ========================================== Files 35 35 Lines 4793 4785 -8 ========================================== + Hits 4205 4210 +5 + Misses 588 575 -13 ```

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