Improve whitespace and unary keyword parsing

Improve whitespace and unary keyword parsing

* lisp/progmodes/js.el (js--name-start-chars): Remove, adding these
chars back to js--name-start-re.
(js--name-start-re): Add chars back from js--name-start-chars.

(js-jsx--tag-start-re): Improve regexp to capture the tag name (so it
can be disambiguated from a unary keyword), to match newlines (which
are common in this spot), and to require at least one whitespace
character before the attribute name.

(js-jsx--matched-tag-type): Ensure the “tag name” isn’t possibly a
unary keyword.

(js-jsx--self-closing-re, js-jsx--matching-close-tag-pos): Allow
whitespace around “<” and “>”.

* test/manual/indent/jsx-unclosed-2.jsx: Add tests for unary keyword
and whitespace parsing.