refactor(theme-classic): migrate to tsc for build

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

This PR removes the Babel CLI used to build & watch the classic theme. There are three reasons.

1. I have constantly talked about using tsconfig's paths option to replace our custom ambient modules declaration. In order to do so, we have to let tsc emit declaration anyways—we can't only use it for type-checking.
2. This is the only place in our code base that uses Babel. Everywhere else uses SWC. Removing it enables us to remove a few dependencies.
3. I've just fixed the watch DX issue (misc: make copyUntypedFiles work for watch mode #7445). This means that CSS gets hot reloaded, and the last reason to use Babel is gone.

Performance is a non-issue. Again, we have to run tsc anyways for type-checking. Chaining more commands only decreases performance.

I've verified that the output of tsc is still human-readable enough (despite missing a few line endings—Babel suffer from the same issue).

This means we no longer have lib and lib-next as two build outputs—everything's in one output folder, just the same as every other package.

Test Plan

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 74	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 86	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: -332 B (0%)

Total Size: 796 kB

Filename	Size	Change
website/build/assets/js/main.********.js	599 kB	-332 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	106 kB
website/build/index.html	38.9 kB

_{compressed-size-action}

[Josh-Cena]

Every time we preserve the ESM client module build output, we get a bundle size decrease! 😄

Same as the size decrease in #7437. Previously, we compiled the client modules to CJS as well, because it was really hard to do fine-grained include/exclude back then without project references. It's good to let Webpack take care of everything!

[slorber]

nice 👍 thanks

maybe we could run Prettier on emitted theme files if anyone complains

[Josh-Cena]

We already do run Prettier; the only thing that can't be recovered is empty lines, but absence of empty lines do subtract readability.

[slorber]

I see

Does not look like a big deal

We could write a little script that ads \n quite easily to top-level export and functions, should be good enough

[Josh-Cena]

I think we can enable newline-after-import and run the fixer or something like that... 🤔

Gonna defer this to the future; the build script is already getting bloated without #5612