KaTeX support

[MoshiBar]

katex support, for math typesetting

What

add katex support to allow for proper displaying of math expressions

Why

so that math expressions can be properly shown (feature parity with firefish)

Additional info (optional)

example: before change after change

Checklist

• [x] Read the contribution guide
• [x] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [ ] (If needed) Update CHANGELOG.md
• [ ] (If possible) Add tests