[PR] Return union void for some string functions

[Brianzchen]

Closes #8284

codePointAt and ''[123] has the potential to return undefined. This change will enforce users to put guards around these use cases to provide safer string manipulation.

[Brianzchen]

For my index test I had to write it as (new String()[123]: string); because (''[123]: string) wasn't throwing an error. Not sure if I'm missing something here because the previous test that uses string literal works fine.

[nnmrts]

@Brianzchen
Looks good, also thanks for the parameter name changes.

I don't really care about flow anymore these days though, sorry.

[Brianzchen]

I've removed the index of check to make this PR a bit easier to get in. Flow team has already expressed that is an area of unsoundness that they are prepared to allow for an easier dev experience.

@nnmrts I understand, hope things get better and you feel interested again soon 😄

[nnmrts]

Reviewed the last commits but I don't know if this does anything or is even relevant anymore.

[Brianzchen]

I think so, the definition if it hasn't been corrected is in fact wrong.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/codePointAt#return_value

It's documented as

If index is out of the range of 0 – str.length - 1, codePointAt() returns undefined.