feat!: Convert to ESM

[nzakas]

In preparing for renaming to @eslint/markdown, I went through and converted the plugin to ESM format. This includes:

• Updating dependencies, including mdast-util-from-markdown, to support ESM.
• Updating package.json to work correctly
• Switching from nyc to c8 for code coverage
• Updating release-please.yml to include an optional run of npm run build (in preparation for adding types)
• Renamed lib to src to better match our newer repos
• Renamed tests to include .test.js to make them easier to glob

Depends on #258

[mdjermanovic]

LGTM, thanks!