Add support for Jekyll collections to jekyll-optional-front-matter

[Copilot]

This PR extends the jekyll-optional-front-matter plugin to support Jekyll collections, addressing the limitation that it previously only worked with regular pages.

Problem

The plugin only processed markdown files in the site root as pages, but didn't handle collection documents (files in _posts/, _articles/, etc.) that lacked front matter. Users wanting to use the plugin with collections had to manually add front matter to every collection document.

Solution

Extended the generator to:

• Detect markdown files in collection directories that lack front matter
• Convert these static files into proper Jekyll::Document objects
• Add them to their respective collections with content pre-converted to HTML
• Optionally remove the original static files when remove_originals is enabled

Key Changes

• Enhanced generator logic: Now processes both pages and collection documents
• New collection detection: Identifies collection directories and validates against site configuration
• Document creation: Converts static files to Jekyll::Document objects with proper content loading
• Blacklist compatibility: Extended filename blacklist to work with both pages and documents
• Content conversion: Pre-converts Markdown to HTML for collection documents
• Static file cleanup: Properly handles removal of original collection files

Example Usage

# _config.yml
collections:
  articles:
    output: true
    permalink: /:collection/:name/

optional_front_matter:
  remove_originals: true

<!-- _articles/my-article.md (no front matter needed) -->
# My Article

This will now be processed as a collection document.

Testing

Added comprehensive tests covering:

• Collection document detection and processing
• Integration with existing page functionality
• Static file removal for collections
• Blacklist functionality for collection documents
• Multiple collection types (posts, custom collections)

All existing tests continue to pass, ensuring backward compatibility.

Fixes #41.

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