feat: switch to svelte-markdown

This was actually used in early versions of photon, but I thought it had a vulnerability. no, i was just stupid.

This renderer allows me to use my own svelte components instead of native HTML ones, which allows me to do more complex stuff than just change styling.

For example, this allows:

expanding images on click
embedding more forms of media in comments
better spoiler element
custom codeblock element, which you can copy text from easily
- in the future, i can add syntax highlighting if i can find a lightweight way to do it

Xyphyn / photon

feat: switch to svelte-markdown #396