switch from babel-sublime JS syntax to sublime-JS-custom

[emidoots]

Today we use babel-sublime for jsx support (see https://github.com/sourcegraph/syntect_server/issues/3) but babel-sublime is not well-maintained anymore, and upon updating sublimehq/Packages it appears it is much more incompatible with the base Packages now and e.g. breaks JS embedded in PHP/HTML.

https://github.com/sublimehq/Packages/issues/860 makes note of a new project, https://github.com/Thom1729/Sublime-JS-Custom, which appears to add JSX on top of the base JavaScript syntax and may be better in the near term.

[emidoots]

Appears to work quite well:

Only annoying part is it's not pre-generated for us to include, but that's easy to fix: https://github.com/slimsag/Sublime-JS-Custom-Generated