ctrl+m and ctrl+e support for [math] and [eqn] tags.

[ekce]

The extension.js file has site-wide ctrl+s support for [spoiler] tags. It would be both useful and intuitive to implement the same type of support for [math] and [eqn] tags on /sci/ since many LaTeX editors already have very similar ctrl+key functionality for "math mode". It is currently very tedious to write something requiring lots of separate math tags.

The relevant code in the extension.js file starts at 1749 and ends at 1768.