Properly set a dynamic, syntactic mode name

Properly set a dynamic, syntactic mode name

Use mode-line-format constructs to properly set mode-name, rather than
use the very hacky solution that was filling-in for my lack of
knowledge of this feature.

* lisp/progmodes/js.el (js--update-mode-name)
(js--idly-update-mode-name): Remove.

(js--syntactic-mode-name-part): New helper function for mode-name.
(js-use-syntactic-mode-name): Helper to set up the dynamic mode-name.

(js-jsx-enable): Don’t need to call any extra functions now.
(js-mode): Use the new setup function rather than the old ones.

(js-jsx-mode): Use the same initial mode name as js-mode so the final
one is identical for both modes.