previously language syntax used TextMates xml based tmLanguage files for syntax highlighting.

Example: https://github.com/Benvie/JavaScriptNext.tmLanguage/blob/mas...

Well, sure it 'uses" xml bases representation, but you can actually write the syntax in JSON and Sublime Text will compile it down to XML.

Example: https://github.com/Nessphoro/sublimeassembly/blob/master/Ass...

Output: https://github.com/Nessphoro/sublimeassembly/blob/master/Ass...

https://github.com/SublimeText/AAAPackageDev lets you create it with YAML. If nothing else, not having to escape and quote your regular expressions seems like a winning proposition.

How? What do you run to write out XML to disk?

I suspect you're confusing what some juiced-up workflow makes possible vs. how stock Sublime works.

Pushing and popping a context is a pretty big deal.