feat: introduce new command check-unnecessary-nullable

[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: new command

New command to check if a parameter of a function, a method or a constructor is marked nullable, but used everywhere as non-nullable. Removing nullability from such parameters can remove additional nullable checks and reduce the complexity of code.

[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: 445

• Total lines of source code: 7944

• Total classes: 308

• Average Cyclomatic Number per line of code: 0.35

• Average Source Lines of Code per method: 6

• Total tech debt: 1370.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #874 (d50a1ea) into master (10c3162) will decrease coverage by 0.84%.
The diff coverage is 71.85%.

❗ Current head d50a1ea differs from pull request most recent head 476ced1. Consider uploading reports for the commit 476ced1 to get more accurate results

@@            Coverage Diff             @@
##           master     #874      +/-   ##
==========================================
- Coverage   87.45%   86.60%   -0.85%     
==========================================
  Files         283      293      +10     
  Lines        6033     6213     +180     
==========================================
+ Hits         5276     5381     +105     
- Misses        757      832      +75     

Impacted Files	Coverage Δ
..._list/json/unnecessary_nullable_json_reporter.dart	0.00% <0.00%> (ø)
.../reporters/unnecessary_nullable_report_params.dart	0.00% <0.00%> (ø)
...console/unnecessary_nullable_console_reporter.dart	4.00% <4.00%> (ø)
...i/commands/check_unnecessary_nullable_command.dart	55.81% <55.81%> (ø)
...cessary_nullable_analyzer/invocations_visitor.dart	56.25% <56.25%> (ø)
lib/src/config_builder/config_builder.dart	81.48% <75.00%> (-1.13%)	⬇️
...nullable_analyzer/unnecessary_nullable_config.dart	83.33% <83.33%> (ø)
..._nullable_analyzer/reporters/reporter_factory.dart	85.71% <85.71%> (ø)
...llable_analyzer/unnecessary_nullable_analyzer.dart	98.00% <98.00%> (ø)
...essary_nullable_analyzer/declarations_visitor.dart	100.00% <100.00%> (ø)
... and 19 more

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