polish(v2): use npm-to-yarn for the npm2yarn remark plugin

[nebrelbug]

Motivation

Docusaurus' previous NPM-Yarn code tab generation was very basic and missed many conversions. I extracted some of the logic into a new npm package (npm-to-yarn) and made the code more robust.

Have you read the Contributing Guidelines on pull requests?

Yep

Test Plan

(Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work. Bonus points for screenshots and videos!)

When serving on localhost, NPM to Yarn code tab generation works in almost all cases (though for some reason, it seems to fail, as did the last implementation, on one-line codeblocks? I think it's a problem with the Remark plugin)

Actually, the problems I observed previously aren't there any more. Conversions seem to be working perfectly

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

None yet

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Deploy preview for docusaurus-2 ready!

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[nebrelbug]

Strangely, the bottom two npm2yarn codeblocks on the Installation page do not convert when running a local server on my device, but do convert correctly on the deploy preview (which I can confirm is using my changes).

This is with the most recent changes, and testing both /docs/ and /docs/next

[nebrelbug]

After fixing a few bugs in my NPM package, I can confirm that I didn't see any failed conversions while browsing the docs (except the codeblock that says npm docusaurus --version, which isn't actually correct)

[yangshun]

Woohoo thank you!