Fix extend_config support for lint_roller-based RuboCop plugins

[HolyGrail]

Problem

RuboCop plugins migrated to lint_roller, but extend_config doesn't process the plugins: directive to load and register plugins dynamically.

Resolves #701 and #715.

Solution

Automatically detect and integrate lint_roller plugins when:

1. Main config declares plugins - plugins: [rubocop-xxx] in .standard.yml
2. extend_config contains plugins + namespaced cops - Both plugins: section and cop configurations in extended files

This preserves backward compatibility while enabling dynamic plugin loading through extend_config.

Technical Approach

• Plugin detection: Scan gems for default_lint_roller_plugin metadata
• Conditional activation: Only when explicit plugin usage detected
• Aligns with RuboCop direction: Supports plugins: (not deprecated require:)

Implementation Details

• Added DetectsLintRollerPlugins to discover lint_roller plugins from loaded gems
• Enhanced MergesUserConfigExtensions with conditional detection logic
• Uses permit_merging for proper cop configuration merging
• Maintains first-in-wins semantics for configuration precedence

Testing

Verified with demo repository that reproduces the original issues:
https://github.com/andyw8/demo-standard-rubocop-minitest

Before:

$ standardrb test/example_test.rb
# No output (offense not detected)

After:

$ standardrb test/example_test.rb
standard: Use Ruby Standard Style
  test/example_test.rb:6:18: Minitest/LiteralAsActualArgument: Replace the literal with the first argument.

Author's Note

This pull request was created with AI assistance to resolve the reported issues. I apologize if extending extend_config functionality doesn't align with the project's philosophy.
I use the standard gem as part of my work at my company, and I hope this project will continue to actively address such issues and keep pace with frequent RuboCop gem updates. I'd be happy if this contribution helps in some way.
Thank you.