Add allow_underscore_prefixed_names option to unused_parameter

[theamodhshetty]

Summary

This adds a new unused_parameter configuration option:

• allow_underscore_prefixed_names (default: false)

When enabled, unused parameters whose names start with _ are treated as intentionally unused and won't trigger violations.

Changes

• Added UnusedParameterConfiguration with:
  • severity
  • allow_underscore_prefixed_names
• Updated UnusedParameterRule to use the new config and skip violations for _-prefixed names when enabled.
• Added rule examples for default and configured behavior.
• Added configuration parsing test.
• Added default config entry in integration fixtures.
• Added changelog entry under Main > Enhancements.

Testing

• SWIFTPM_ENABLE_KEYCHAIN=0 swift run swiftlint lint Source/SwiftLintBuiltInRules/Rules/Lint/UnusedParameterRule.swift Source/SwiftLintBuiltInRules/Rules/RuleConfigurations/UnusedParameterConfiguration.swift Tests/FrameworkTests/RuleConfigurationTests.swift
• SWIFTPM_ENABLE_KEYCHAIN=0 swift test --filter UnusedParameterRuleGeneratedTests
• SWIFTPM_ENABLE_KEYCHAIN=0 swift test --filter RuleConfigurationTests

Closes #5741

[theamodhshetty]

Aligned this with discussion in #5741 by keeping behavior unchanged by default and making underscore-prefixed names opt-in via allow_underscore_prefixed_names.

Happy to adjust naming or scope if maintainers prefer a different config shape.

[SwiftLintBot]

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[SimplyDanny]

Thank you for the contribution!