Add support for importing math expressions from Markdown

[dousha]

The ability to typeset math expressions in markdown is now considered essential. Many other markdown-based note-taking software supports using TeX-like expressions for this, as demonstrated in #4387.

This PR adds proper support for importing math expressions from Markdown by leveraging the extension functionality of marked. A set of tokenizers is added to marked to parse these expressions and convert them into KaTeX-compliant HTML output.

Closes #4387.

[dousha]

Should #4417 be merged, we can just switch to using marked-katex-extension, which lists katex as a dependency.

[zadam]

@dousha #4417 has been merged. Do you need to modify your PR now?

[dousha]

There are some modifications made to the original marked-katex-extension. Notably the ability to use \( \) and \[ \] for math expressions. I'll merge these modifications to marked-katex-extension first.