Use JavaScriptNext as the base for JSX syntax highlighting

Not sure if this is a _good idea_, but it's worth considering:

I've combined [Benvie/JavaScriptNext](https://github.com/Benvie/JavaScriptNext.tmLanguage) (which has really good ES6 support) with [yungsters/sublime](https://github.com/yungsters/sublime)'s for JSX support. Since the React community tends to write ES6 flavored JSX, it makes sense to have good ES6 support.

I took [these lines](https://github.com/yungsters/sublime/blob/10980f0/tmLanguage/JavaScript%20(JSX).JSON-tmLanguage#L450-L558) from @yungsters version. Only changing the `jsx-evaluated-code` section so it included `#core` instead of `#expression`. And added them to @benvie's [Benvie/JavaScriptNext.tmLanguage](https://github.com/Benvie/JavaScriptNext.tmLanguage/blob/41c1b8b/JavaScriptNext.YAML-tmLanguage), as the diff in the comment shows.

I'm not sure if @benvie would even be ok with this (there is no license info). I'm not sure if this is 100% correct, but from cursory use, it seems to work just fine. Maybe someone with more tmLanguage experience can vet this?