docs: add onlyRemoveTypeImports to (preset|plugin-transform)typescript

[kaicataldo]

Refs babel/babel#11173 (comment)

This PR adds documentation for the new onlyRemoveTypeImports configuration option in the upcoming 7.9.0 release.

Questions:

1. This is my first time working in this repo. Did I miss anything?
2. I fixed up the docs so that they matched and did a little editing. Is it okay that I didn't backport these changes to the older versions of the docs?

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[existentialism]

I fixed up the docs so that they matched and did a little editing. Is it okay that I didn't backport these changes to the older versions of the docs?

It's not critical, but I personally think we should backport :)

[kaicataldo]

@existentialism Which parts do you think we should backport? Everything but the new option?

[nicolo-ribaudo]

We should also add the equivalent tsc option to the "TypeScript Compiler Options" section.

[kaicataldo]

Does this seem correct/clear?

[nicolo-ribaudo]

Maybe we can't mention that onlyRemoveTypeImports: false isn't 100% accurate because we don't do any type analysis, and link to babel/babel#8361 as an example? Also, it's the reason why we don't support importsNotUsedAsValues: error.

[kaicataldo]

Do you mind clarifying what you mean?

[nicolo-ribaudo]

Sure!

Consider this example:

// dep.ts

export type T = string;

// main.ts

import { T } from "./dep";
export { T };

onlyRemoveTypeImports: false leave main.ts as-is, because we don't have the type information necessary to understand that T is a type.
However, TypeScript's importsNotUsedAsValues: remove transforms it to an empty file because it knows that T is a type.

[kaicataldo]

Thanks for the comments! Ready for re-review :)

[kaicataldo]

Thanks for updating this!