Allow multiline string literals as JSX attribute values.

[mjambon]

Typescript supports unescaped newlines within string literals used as jsx attributes, like this:

<a b="hello
world">

(same code in typescript playground)

This PR adds supports for unescaped newlines in tsx attribute values.

Another, simpler solution would be to directly modify the definition of $.string in the javascript grammar. Having unescaped newlines in any string literal would not be legal javascript or typescript (outside of tsx attributes) but I don't see a big risk in doing so.

[patrickt]

Having unescaped newlines in any string literal would not be legal javascript or typescript (outside of tsx attributes) but I don't see a big risk in doing so

We tend to err on the side of simpler grammars rather than strict conformance, so this seems like a good idea if it makes the grammar simpler and (hopefully) smaller.

[maxbrunsfeld]

Yeah, I'm guessing that if it's allowed in TSX, it may be allowed by some (non-typescript) JSX preprocessors like Babel. So I think it would make sense to just add this in the JavaScript grammar, either by modifying string, or creating a special jsx_string rule (which could be aliased as string).

[mjambon]

Great, I'll move this to javascript (as a jsx_string, probably)