New Markdown layout props API

[bholmesdev]

What kind of changes does this PR include?

• New or updated content

Description

• Change "Markdown props" -> "Markdown layout props" for clarity
• Remove .astro object from Markdown
• Introduce new props alongside content: headings, rawContent, and compiledContent
• Update old getHeaders -> getHeadings

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[sarah11918]

Thanks @bholmesdev ! I will look at this, but FYI there's an existing PR re: Headers -> Headings with problems that we've had sitting here for a while. Do you mind taking a peek at that one while I look at this one to see if its existence is relevant here?

#1107

[bholmesdev]

@sarah11918 Ah yes, there will be conflicts with that draft! Looks like @natemoo-re is updating a few references that were removed in this PR. I'd suggest getting this one in first and updating Nate's accordingly.

[sarah11918]

Thanks @bholmesdev! Just a couple of quick nits to check on!