improve types for `String.split`

[ivanhofer]

This PR improves the type definitions for String.split. By default it always returns string[] but depending on the arguments passed, we can infer a few cases e.g.

• Splitting a string will almost always return an array with at least one item, so the type [string, ...string[]] makes more sense.
• Unless when the string and separator is an empty string '', then an empty array [] is returned. In that case string[] makes sense again
• When the limit is set to 0 an empty array [] gets returned.

[lo1tuma]

FWIW: there is also on open issue in the typescript repo, which hasn’t been closed yet. So there is a chance that this will be supported upstream.

That’s being said, I would be very happy about such an improved version of String.prototype.split. No matter if it is in typescript directly, or here via ts-reset.

[mattpocock]

Thanks for the PR! I won't merge it, for reasons given here.