BREAKING CHANGE(gatsby-plugin-mdx): MDX v2

[axe312ger]

This PR replaces the old gatsby-plugin-mdx with a new version that is powered by MDX v2.

We won't be 100% backwards compatible, but most use cases will be covered. Simplification and better performance is the overall goal.

Separate PRs

• mdx-v2: feat: add support for remark, rehype and gatsby-remark plugins #35751
• mdx-v2: remove mdx from graphql #35752
• mdx-v2: remove old code #35753
• MDXv2: codemods #35888
• feat(gatsby-plugin-mdx): add ToC and excerpt support #35873
• MDXv2: remove default layouts and add support for pageQueries #35983
• chore(docs): MDX v2 #35893
• MDXv2: update example using-mdx #36009
• MDXv2: new starter #36201
• MDX v2: Support FS routes #36118

Changes:

This is a complete rewrite of the plugin, so some old features might be gone.

New features

• Upgraded all dependencies including MDX to v2
• Per file tree-shaking and chunking via webpack
• MDXRenderer is no more needed.
• Similar or better performance than old plugin. (First tests on complex projects show similar performance)
• Extended e2e tests
• Simplified plugin configuration
• Via the mdxOptions config option you can pass any configuration to @mdx-js/js compile like remarkPlugins, rehypePlugins, recmaPlugins and more.
• Move away from react-test-renderer in tests
• Authored in TS

Kept features

• .mdx files from src/pages are turned into real pages
• .mdx files loaded via gatsby-source-filesystem or as component via createPage can be turned into real pages
• support for frontmatter (via GraphQL and page context)
• excerpt & table of contents support feat(gatsby-plugin-mdx): add ToC and excerpt support #35873
• support for remark, rehype and most gatsby-remark-* plugins mdx-v2: feat: add support for remark, rehype and gatsby-remark plugins #35751

Removed features

• (compiled) MDX code won't be available anymore within GraphQL
• MD nodes are no more automatically turned into MDX
• config option defaultLayouts is gone
• config option lessBabel is gone
• config option mediaTypes is gone
• config option shouldBlockNodeFromTransformation is gone
• some GraphQL data is no more available in core and have to be implemented on project level (like time to read)

Post Release ToDos

• Investigate if we could/should replace gray-matter with the recommended approach by MDX via remark-mdx-frontmatter

Closes #25068
Closes #34421
Closes #35733
Closes #35732

[LekoArts]

TODOs from other PR:

• mdx-v2: feat: add support for remark, rehype and gatsby-remark plugins #35751 (comment)
• mdx-v2: feat: add support for remark, rehype and gatsby-remark plugins #35751 (comment)
• mdx-v2: feat: add support for remark, rehype and gatsby-remark plugins #35751 (comment)

[alvinometric]

I'm so excited about this! Great work @axe312ger 👏 👏