Add option to validate links

[Copilot]

This PR adds a new configuration option validate_links to the Jekyll Relative Links plugin. When enabled, it provides functionality similar to Jekyll's native {% link %} tag by validating that linked files actually exist.

Features

• Added new validate_links configuration option (default: false for backward compatibility)
• When enabled, the plugin will raise an error when a link points to a nonexistent file
• Added documentation in README explaining the new feature
• Added tests to verify validation behavior

Usage

To enable link validation, add this to your _config.yml:

relative_links:
  validate_links: true

With this option enabled, Jekyll will raise an error during build if any relative link points to a nonexistent file, helping to prevent broken links in your site.

Fixes #77.

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[benbalter]

@copilot can you fix the failing tests?

[benbalter]

@copilot can you fix the failing tests?

[Copilot]

@copilot can you fix the failing tests?

I've fixed the failing tests by addressing the RuboCop violations. The main issues were:

1. Trailing whitespace in both source and test files
2. Minor style issues in the test code for regular expressions and block syntax
3. A pre-existing complexity issue in the replace_relative_links! method

All tests are now passing. The commit that fixes these issues is 6440423.