Add remark-directive-babel8-plugin

[JLHwung]

In this PR we introduced a remark-directive-babel8-plugin. It is essentially a BABEL_8_BREAKING toggle (like we introduced in the main repo) working on the markdown docs:

When process.env.BABEL_8_BREAKING is 1, everything within :::babel8 and ::: will be rendered while everything within :::babel7 and ::: will be discarded. Otherwise their behaviour is swapped.

In this way we can maintain both the Babel 7 and Babel 8 docs in the same branch.

Due to the AST limit of MDX v1 (Docusaurus 3 will use MDX v2), the plugin has to hijack the paragraph AST node and manually check for :::babel8. In MDX v2 we can hook on the ContainerDirective AST type directly.

Preview link: https://deploy-preview-2772--rad-sopapillas-370b65.netlify.app/docs/babel-preset-typescript#options (both isTSX and allExtensions are removed in Babel 8)

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[nicolo-ribaudo]

I like this! We could then publish next.babeljs.io.

One alternative would be to have a client-side toggle that shows/hides the verison-specific sections.

[JLHwung]

One alternative would be to have a client-side toggle that shows/hides the verison-specific sections.

The current remark plugin can deal with most differences in markdown docs. But we will have more to take care, such as sidebar items, REPL settings. A client toggle will complicate the implementation when we introduced more Babel 8 changes. Therefore I lean towards the current approach, we can publish the Babel 8 changes to, like you said, next.babeljs.io.

[JLHwung]

The docs changes are removed in this PR. They will be fit into separate PRs so that we can link the docs PR to the actual change PR.

[JLHwung]

CI is failing because we hit the open collective API rate limit using two netlify builds:

The API is public and you can freely use it up to 10 request / minute without authentication.

https://graphql-docs-v2.opencollective.com/access#authentication

I will see if we can migrate to a personal token easily or we can just disable the sponsors downloading on Babel 8 netlify builds.