[MDX] Support YAML frontmatter

[bholmesdev]

Changes

• Add 2 new remark plugins: remark-frontmatter, remark-mdx-frontmatter
  • Apply both plugins regardless of the user's remarkPlugins config. This is different from GFM and smartypants, which are each removed when configuring without extends. Let me know if this should be changed!
• Apply frontmatter to frontmatter object export by default. This should make the Astro.glob experience fall in-line with plain markdown!
• Add frontmatterOptions to config. This allows users to override remark-mdx-frontmatter defaults (ex. use a name other than the frontmatter object as the destination for YAML, including name: '' to avoid our frontmatter object).

Testing

• Test frontmatter object
• Test glob result
• Test custom frontmatter name config

Docs

• add variables and frontmatter explainers to show both use cases
• add frontmatterOptions to README

[changeset-bot]

🦋 Changeset detected

Latest commit: 63e3074

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@astrojs/mdx	Minor
astro	Patch
@e2e/preact-component	Patch
@e2e/react-component	Patch
@e2e/solid-component	Patch
@e2e/svelte-component	Patch
@e2e/astro-component	Patch
@e2e/error-react-spectrum	Patch
@e2e/error-sass	Patch
@e2e/errors	Patch
@e2e/lit-component	Patch
@e2e/e2e-tailwindcss	Patch
@e2e/ts-resolution	Patch

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[bholmesdev]

remark-mdx-frontmatter applies export const frontmatter = undefined when no frontmatter is present. This handles that case!

[natemoo-re]

This looks good! Is there any particular reason to expose the RemarkMdxFrontmatterOptions to the user? If it's only to change frontmatter, I feel like we probably don't need to expose it? I don't think we've ever gotten any requests to configure this setting for .md files.

[bholmesdev]

@natemoo-re Added some use cases to the README! I'd argue this is more necessary for MDX, since all exports are at the root level by default. I could see people wanting to put YAML frontmatter at the root as well 🤷
https://github.com/withastro/astro/pull/3995/files#diff-f8b3ea982e3a2f14022902aeaefe786a2477d13f756d4679f70a249b2bacfd7bR204-R206