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]

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 ⚠️

Additional details and impacted files

@@            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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