Added separate JSXText type; emitter enhancements.

* Added separate JSXText with own emitter with support for emitting multiline JS strings with pre-escaped special and Unicode characters.
* Added own emitter for JSXExpressionContainer with support for empty containers (emitting inner comments if available) and no-parenthesis output.
Implemented contextual type for JSXElement children as args.
Small style/semantic fixes.
Fixed test: empty JSX container should emit it's comments.