Propertize and font-lock JSXText and JSXExpressionContainers

Propertize and font-lock JSXText and JSXExpressionContainers

This completes highlighting support for JSX, as requested in:

- https://github.com/mooz/js2-mode/issues/140
- https://github.com/mooz/js2-mode/issues/330
- https://github.com/mooz/js2-mode/issues/409

* lisp/progmodes/js.el (js--name-start-chars): Extract part of
js--name-start-re so it can be reused in another regexp.
(js--name-start-re): Use js--name-start-chars.

(js-jsx--font-lock-keywords): Use new matchers.
(js-jsx--match-text, js-jsx--match-expr): New matchers to remove
typical JS font-locking and extend the font-locked region,
respectively.

(js-jsx--tag-re, js-jsx--self-closing-re): New regexps matching JSX.
(js-jsx--matched-tag-type, js-jsx--matching-close-tag-pos)
(js-jsx--enclosing-curly-pos, js-jsx--enclosing-tag-pos)
(js-jsx--at-enclosing-tag-child-p): New functions for parsing and
analyzing JSX.

(js-jsx--text-range, js-jsx--syntax-propertize-tag-text): New
functions for propertizing JSXText.
(js-jsx--syntax-propertize-tag): Propertize JSXText children of tags.
(js-jsx--text-properties): Remove JSXText-related text properties when
repropertizing.
(js-mode): Extend the syntax-propertize region with
syntax-propertize-multiline; we are now adding the syntax-multiline
text property to buffer ranges that are JSXText to ensure the whole
multiline JSX construct is reidentified.