support TS 4.3 static index signature in class

[g-plane]

Q	A
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
License	MIT

Reference: https://devblogs.microsoft.com/typescript/announcing-typescript-4-3-beta/#static-index-signatures

Allow:

class Foo {
    static [propName: string]: string | undefined;
}

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/45050/

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[nicolo-ribaudo]

For the tests failure, you can run make test-typescript-update-allowlist.

[nicolo-ribaudo]

Thanks!

After that this PR is merged, would you then like to open a new PR to use the InvalidModifiersOrder error for

class A {
  readonly static foo = 2;
}

?

Currently it just throws "unexpected token" (repl).

[nicolo-ribaudo]

Oh also, could you rebase this on top of https://github.com/babel/babel/tree/feat-7.14.0/parser-updates? 🙏

I'm using that branch for all the parser PRs for 7.14.0, to avoid merge conflicts when we'll merge all the PRs for the release.

[nicolo-ribaudo]

make test-typescript-update-allowlist 🙏