Consistent MD frontmatter typing

[mrienstra]

Changes

• MarkdownAstroData["frontmatter"] was too vague (object), now using MarkdownInstance
• MarkdownParserResponse uses MarkdownInstance (minor)
• I was going to go with something more like:

  export type MarkdownAstroData<T extends Record<string, any>> = {
      frontmatter: MarkdownInstance<T>['frontmatter'];
  };

  ... but I didn't want to make those generic (breaking change).

Testing

n/a, type changes only

Docs

n/a, type changes only

[changeset-bot]

🦋 Changeset detected

Latest commit: 505e78a

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[matthewp]

@mrienstra needs a changeset, thanks!

[matthewp]

Thank you!