Allow 3rd party plugins to filter frontmatter

[webdados]

I'm developing a small Markdown Alternate with WooCommerce at https://github.com/webdados/markdown-alternate-woocommerce

Because a WooCommerce product does not have the category and post_tag taxonomies, I need to be able to manipulate the frontmatter to:

• Remove the post taxonomies
• Add the product taxonomies

Ideally, there should exist a filter that allows 3rd party developers to manipulate the frontmatter to add/remove lines.

[webdados]

I've refactored ContentRenderer::generate_frontmatter at https://github.com/webdados/markdown-alternate/blob/refactor-frontmatter/src/Output/ContentRenderer.php#L76 as an exercise in making it more flexible and filterable for 3rd parties.

Here's what was implemented:

• Separated the concept of $lines and $content_lines
  • $lines are the raw lines that will be imploded into plain text
  • $content_lines is an associative array with all the lines that actually have content; the key is the name of the content, and it can hold two different types of lines: content (for single lines) or items (for lines with several items, like taxonomies)
  • There's also a type array entry for the content so that we can format it for YAML (with or without quotes); If type = array, then we expect it to be a multiple line content, like taxonomies, and have an items entry
• Content lines are filterable, so a 3rd party developer can remove lines by their name, or add new lines
• The taxonomies are now also filterable. The array key is the WordPress taxonomy, and the value is the name that is used for the output
• New format_yaml_value method to add / not add quotes to the values based on their type
• The content lines are added to the raw lines, and those are also filterable

I didn't submit a PR because I wanted you to review the implementation first and tell me whether this approach makes sense and what should be improved to better align with your codebase and architecture. For example, I'm not 100% sure of the "content vs. items" for content lines; this was just a quick implementation to test it with my WooCommerce integration.

Here's the current output for a WooCommerce product, using my version of frontmatter:

Let me know if this approach makes sense, would be something you would be interested in merging, and what you would change, so I can harden it before I submit a PR.

[webdados]

I just tweaked it a little so that type = array controls either each content line is a single or multiple raw lines, and updated the description above.