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:

• [x] https://github.com/eslint/eslint-plugin-markdown/pull/258
• [x] https://github.com/eslint/eslint-plugin-markdown/pull/259
• [x] https://github.com/eslint/eslint-plugin-markdown/issues/255
• [x] Rename to @eslint/markdown
• [x] Rename repo to eslint/markdown
• [x] Create markdown language
• [x] 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! :tada: