feat: add monorepo flag for check-unused-files command

[incendial]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[X] Other, please explain: add monorepo flag for check-unused-files command to mark exports as unused by default.

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 390

• Total lines of source code: 6921

• Total classes: 259

• Average Cyclomatic Number per line of code: 0.35 / 1

• Average Source Lines of Code per method: 6

• Total tech debt: 1126.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #684 (fa0a196) into master (f81d54a) will increase coverage by 0.09%.
The diff coverage is 81.08%.

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
+ Coverage   86.35%   86.45%   +0.09%     
==========================================
  Files         255      255              
  Lines        5439     5448       +9     
==========================================
+ Hits         4697     4710      +13     
+ Misses        742      738       -4     

Impacted Files	Coverage Δ
...ib/src/cli/commands/check_unused_code_command.dart	55.81% <ø> (ø)
lib/src/config_builder/config_builder.dart	82.60% <0.00%> (ø)
...b/src/cli/commands/check_unused_files_command.dart	58.00% <62.50%> (+2.18%)	⬆️
...s/unused_files_analyzer/unused_files_analyzer.dart	82.05% <83.33%> (-0.45%)	⬇️
lib/src/utils/analyzer_utils.dart	88.23% <85.71%> (-1.77%)	⬇️
...ers/unused_code_analyzer/unused_code_analyzer.dart	93.75% <100.00%> (+0.64%)	⬆️
...ers/unused_files_analyzer/unused_files_config.dart	100.00% <100.00%> (+20.00%)	⬆️
...rs/unused_files_analyzer/unused_files_visitor.dart	100.00% <100.00%> (ø)
...yzers/unused_code_analyzer/unused_code_config.dart	100.00% <0.00%> (+18.18%)	⬆️
... and 1 more

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