Font-lock JSX while editing it by extending regions

Font-lock JSX while editing it by extending regions

* lisp/progmodes/js.el (js-jsx--font-lock-keywords):
Call tag beginning and end matchers.
(js-jsx--match-tag-beg, js-jsx--match-tag-end): New functions.
(js-jsx--syntax-propertize-tag): Record buffer positions of JSXElement
beginning and end for font-locking.

(js--syntax-propertize-extend-region)
(js-jsx--syntax-propertize-extend-region): New functions for extending
the syntax-propertize region backwards to the start of a JSXElement so
its JSXAttribute children on its n+1th lines can be parsed as such
while editing those lines.
(js-mode): Add js--syntax-propertize-extend-region to
syntax-propertize-extend-region-functions.