Convert to @eslint/markdown

[nzakas]

As part of our language plugins work, this package will be renamed to @eslint/markdown over a series of PRs:

• build: Update release-please #258
• feat!: Convert to ESM #259
• Missing TypeScript declarations #255
• Rename to @eslint/markdown
• Rename repo to eslint/markdown
• Create markdown language
• Publish to JSR

[pauliesnug]

The README.md has been updated to use npm install @eslint/markdown, but the NPM package hasn't been published yet

[nzakas]

That's correct.

[nzakas]

All done! 🎉