Fix Markdown errors missing source filename

[hippotastic]

Changes

• Adds a proper source filename to Markdown errors that fail the build.
• Reason: Questions where users did not know which Markdown file was causing a build failure were asked multiple times in our Discord support channel in the last few days.
• Old error message example:

  Expected a closing tag for `<br>` (2:74-2:78) before the end of `paragraph
  

• New error message example:

  Failed to parse Markdown file "D:\Dev\astro-docs\src\pages\en\core-concepts\routing.md":
  Expected a closing tag for `<br>` (2:74-2:78) before the end of `paragraph
  

• Works in build, dev, and SSR modes.

Testing

• Ran all tests locally.
• Performed multiple builds of the Astro Docs repo, inserting test errors in various MD files.

Docs

• Not a visible change, just a bugfix.

[changeset-bot]

🦋 Changeset detected

Latest commit: ec0e6aa

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

This PR includes changesets to release 1 package

Name	Type
@astrojs/markdown-remark	Patch

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[natemoo-re]

LGTM! Thanks for the PR!