chore: refactor ast parser and inspector

[g4s8]

Keep current context for ast.File in AST parser. Iterate over all files in file-set to traverse with AST parser.

For #23

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 44.53961% with 259 lines in your changes missing coverage. Please review.

Project coverage is 43.08%. Comparing base (f8f8c47) to head (591db74).

Files	Patch %	Lines
config.go	0.00%	110 Missing :warning:
parser.go	44.15%	32 Missing and 11 partials :warning:
debug.go	0.00%	40 Missing :warning:
main.go	0.00%	32 Missing :warning:
render.go	0.00%	31 Missing :warning:
converter.go	97.14%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #24       +/-   ##
===========================================
- Coverage   93.89%   43.08%   -50.82%     
===========================================
  Files           9       10        +1     
  Lines         672      564      -108     
===========================================
- Hits          631      243      -388     
- Misses         34      309      +275     
- Partials        7       12        +5     

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[g4s8]

TODO: needs to improve coverage.