Add MarkEdit Textpack Export extension

[haohailong]

Adds markedit-textpack to the official extension registry. The v0.1.0 GitHub Release provides the single-file injectable JS and the entry pins its verified SHA-256. The source repository is public and MIT-licensed.

Validation: extension tests (7/7), registry tests (14/14), and the registry build with full download/integrity checks passed locally. Generated index.json and site files are intentionally not included.

[cyanzhong]

Thanks for contributing this extension! The registry entry and release integrity checks look good, and the basic export functionality works. Just a few edge cases that you may want to address in a future release:

• Escaping asset filenames: Filenames containing parentheses or apostrophes can produce broken Markdown links or single-quoted HTML attributes after rewriting. For example, ![photo](images/a%29.png) becomes ![photo](assets/a).png).
• Matching HTML attributes: When an image has both data-src and src, the exporter can match data-src instead of src, leaving the actual image unbundled.
• Preserving URL fragments: Rewriting a link such as guide.md#introduction drops the fragment, so the exported link no longer points to the intended section.

These are from AI-assisted review, you can validate before taking actions.