previously, a math block would only be recognised when on its own
line

Reviewed-on: https://git.joinsharkey.org/Sharkey/sfm.js/pulls/5

it's not supported by many browsers, also the vue/vite compiler
refuses to allow it

Closes #2

Reviewed-on: https://git.joinsharkey.org/Sharkey/sfm.js/pulls/3

Closes #1

This reverts commit 56002e86.

Fix #129