Support frontmatter in supabase/embeddings-generator

[jmagrippis]

This Github action which is required for https://github.com/supabase/headless-vector-search turns your Markdown or MDX files into usable embeddings. Great!

Frontmatter is very popular with Markdown, especially useful for blogs, so it'd be great if the Github Action could check whether there is any frontmatter, and store it in a separate bson column. We could even have some "magic" fields we could instruct users to optionally use, like title and tags.

As it stands, frontmatter gets chugged in as a page section.

https://github.com/supabase/embeddings-generator