feat: Add low_readability_numeric_literals lint rule and quick fix

[ronnnnn]

Add LowReadabilityNumericLiterals lint rule to check for numeric literals with 5 or more digits without digit separators.

• Lint Rule Implementation

  • Add LowReadabilityNumericLiterals class in packages/nilts/lib/src/lints/low_readability_numeric_literals.dart.
  • Implement run method to check for 5+ digit integers without separators.
  • Implement getFixes method to provide a quick fix.
  • Define _AddDigitSeparators class to add digit separators to 5+ digit integers.

• Integration

  • Add LowReadabilityNumericLiterals to the list of lint rules in packages/nilts/lib/nilts.dart.

• Testing

  • Add test cases for LowReadabilityNumericLiterals lint rule in packages/nilts_test/test/lints/low_readability_numeric_literals.dart.

• Documentation

  • Update packages/nilts/README.md to include low_readability_numeric_literals in the list of lint rules.
  • Provide an overview and details for low_readability_numeric_literals.

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