feat: automatic Markdoc partial resolution

[bholmesdev]

Resolves #10418

Changes

This adds automatic resolution for Markdoc partials by relative file path.

<!--src/content/blog/post.mdoc-->

{% partial file="./_diagram.mdoc" /%}

<!--src/content/blog/_diagram.mdoc-->

## Diagram

<figure>
<img src="..." />
<figcaption>Diagram embedded into <code>post.mdoc</cod>
</figure>

• Crawl Markdoc AST to discover any partial tags. When one is found, render the partial in-place and assign the node. Flattening lets us catch validation errors at buildtime and discover nested component tags for hoisted scripts.
• Ensure manually configured partials are still respected to make this a non-breaking feature
• Track file changes to related partials using addWatchFile()
• Update validator to ignore any "partial not found" errors.

Testing

Add tests to render components and HTML within a partial.

Docs

withastro/docs#7743

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[bholmesdev]

Seems the test file contains too many prod build calls and times out. Will try to reuse fixtures and split the render.test.js file

[sarah11918]

@bholmesdev Just wanted to check on the naming convention here Ben, because we are currently referring to "page partials" as just plain old "partials" (I guess, the only kind we had so it didn't much matter): https://docs.astro.build/en/basics/astro-pages/#page-partials

So, I'm wondering whether in conjunction with your PR documenting Markdoc partials we should update wording on the Pages page to always say "page partial" and whether we should be clear to always say "Markdoc partial" when referring to these files.

The joys of adding the second thing that use the same word. 😄

[ematipico]

Can we have at least one test that raises an error in case we attempt to render a partial that doesn't exist or it's an incorrect folder? This would help to set down expectations.