[sheet] Convert to TypeScript

[sarayourfriend]

What: Converts sheet to TypeScript

Why: Part of #2358

How: Renamed index.js to index.ts and then added back type definitons. Most of them were the same as the flow type save some void -> undefined. Part of that also changed the signature for sheetForTag to return undefined (it technically could) so we need to add a null check. I decided to create a utility called assertDefined similar to the one used by WordPress's dom package: https://github.com/WordPress/gutenberg/blob/af7c07ce630dff9177dd1e25c1459d7e25e836e2/packages/dom/src/utils/assert-is-defined.ts

This allows us to just call assertDefined and have a runtime check that the variable is defined. This runtime check is disabled in production to prevent a performance regression.

I also had to add the private _alreadyInsertedOrderInsensitiveRule attribute to the StyleSheet class. _insertTag is also marked private to match the original .d.ts files.

Also had to update one of the type tests that was erroring out. Not sure why it passed previously as options 100% needs to be defined or the constructor for StyleSheet will throw a TypeError (it doesn't check at all that options is defined).

Overall I think this PR actually adds some type safety/sanity to sheet that flow was missing.

Checklist:

• Documentation N/A
• Tests
• Code complete
• Changeset

Will depend on changes from #2427 for the jest tests to run by figured I'd get it ready anyway

[changeset-bot]

🦋 Changeset detected

Latest commit: 19a5074

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@emotion/sheet	Minor

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[Andarist]

the previous definition was:

constructor(options?: Options);

which was incorrect because options are required.

I'm glad that the whole migration can fix some minor issues like this as isn't done only for the sake of migrating 🎉

[Andarist]

@sarayourfriend thank you for all your hard work on those migrations PRs ❤️ I'm sorry that I didn't get to them sooner - which usually is not a problem - but I have had much less time for OSS lately and my backlog has overwhelmed me.

[sarayourfriend]

@Andarist No worries, I totally understand! Glad I can help 😄