defineMathMode: single $ toggles syntax highlight of all following lines

[cben]

As long as multi-line $...$ is unsupported (#3) should reset to outer mode at EOL.
When fixed, ideally should be configurable but at least stop on empty line?

[cben]

this is soft blocker for using defineMathMode as fix for #4.

[cben]

Another issue with the multiplex addon for parsing markdown/latex:
codemirror/codemirror5#3535
(easily reproduces here too)

I'm increasingly tempted to integrate math (as option) into CodeMirror's markdown mode itself.
However that'd not belong in this project; here I'm interested in fixes that work on top of any mode.