printer: Support \n\n while printing JSXElement children

[JHilker]

The printer seems to treat any number of newlines within a JSXText as a single newline. This causes any transform touching these elements to remove newlines that tooling such as Prettier would otherwise not remove.

Though I'm a little surprised to not see the same formatting difference in ASTExplorer, though it's running on 0.21.1 instead of the latest version (0.23.6), so maybe this was a regression at some point? I can reproduce it both within the test suite, and transforms using 0.23.6.

Edit: Oh, this was because by default the transform itself was using parsers.esprima. It's reproducible if you swap to parsers.babel, example.

Current behavior:

Before	After
const SomeComponent = () => { return ( <div> <Foo /> <Bar /> </div> ); }	const SomeComponent = () => { return ( <div> <FooMutated /> <Bar /> </div> ); }

This PR updates the printer for JSXElement / JSXFragment to return two newlines instead of a single newline for any string children which include two adjacent newlines.

Proposed behavior:

Before	After
const SomeComponent = () => { return ( <div> <Foo /> <Bar /> </div> ); }	const SomeComponent = () => { return ( <div> <FooMutated /> <Bar /> </div> ); }

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Thanks in advance for any review. This seems like an innocent enough change, but I'd entirely understand if it's more complicated than I'm imagining. Open to feedback and happy to iterate on this PR!