Addon-docs: MDX2 support

[shilman]

Issue: #17514 #17137

What I did

• Extract MDXv1 compiler into its own package & clean up snapshot tests
• Create MDXv2 compiler in its own package
• Add opt-in mechanism
• Add MIGRATION docs

How to test

• CI passes
• Battery of snapshot tests

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[tmeasday]

Is there a PR for the two packages we should be looking at?

So this means that all apps will depend on @storybook/mdx1-csf and folks that opt in will depend on both? Is that going to be OK from a dependencies point of view?

[tmeasday]

Console log is intentional?

[shilman]

@tmeasday Yes, there are two more PRs here

• Add MDXv1 compiler, tests, example mdx1-csf#1
• Add MDXv2 compiler, tests, example mdx2-csf#1

The dependency structure is as you described. I don't anticipate any dependency conflicts inside Storybook because each package depends on a specific version and Storybook only depends on the packages. It could, however, cause conflicts with the user's app. This is why we've been prebundling dependencies, and I supppose we could consider prebundling these packages as well. This would require @ndelangen to generalize his prebundling script to be used outside the SB monorepo, and I don't think we need to do it right away.

There is one "hack" which is that SB depends on @mdx-js/react@1.x, which coincidentally works correctly with v2. We could take this PR a step further and also move that dependency into the new packages. However, I didn't do that because:

1. It would complicate things
2. I hope this split will be temporary until 7.0, at which point we'll upgrade to MDXv2 completely

[shilman]

@tmeasday @yannbf Merging this to keep things moving forward & reduce merge conflicts. I'll update the dependencies once the mdx-csf PRs are reviewed/merged.

[agrohs]

@shilman - wondering if this is documented somewhere around how to use the experimental config w/ mdx2 in storybook 6.5 by chance??

(I'm in a large mono repo that across the board moved to mdx2 and really cannot yet move to storybook 7, so hopeful to get this to run/kick some tires on it! Much appreciate in advance for any help/guidance on it)

[agrohs]

Think I may have just found it...is this the correct "guide": https://gist.github.com/shilman/6ff2d7e18db8846e8fc552fb432ae4f6 ??