Update `remark-math` to v3 (#7938)
* bump remark-math to 3.0.1
* escape $ only it was escaped in the input
* remove unsupported syntax from tests
remark-math@3 doesn't support text after the closing fence ($$). Such $$ aren't parsed as a closing fence anymore.
* refactor
* add a test
Co-authored-by: Georgii Dolzhykov <thorn.mailbox@gmail.com>
