MFM support in frontend

[aumetra]

Is your feature request related to a problem? Please describe.

N/A

Describe the solution you'd like

Implementing MFM for Kitsune. Animations are fun.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

To quote @Y0ngg4n:

Unfortunately will probably require a full custom parser for MFM. Still torn on whether I want to implement everything myself at that point, or if I want to just implement the superset features they added on top of markdown, and then just pass the transformed output to pulldown-cmark (the latter is probably the better option)

[tesaguri]

Misskey renders every Note as MFM no matter where it originated, from Misskey or otherwise, which is found controversial by some people (misskey-dev/misskey#15217). Is your intention to follow Misskey's spec and process every post as MFM, or only posts explicitly marked as MFM (e.g. source.mediaType === "text/x.misskeymarkdown")?

Also, even Misskey has end-user options to restrict the range of MFM features to accept (Enable advanced MFM and Enable animated MFM) and I think it'd be sensible to have similar restrictions for obvious accessibility reasons.

[aumetra]

My intention was to only apply the transformation where source.mediaType === "text/x.misskeymarkdown", and for anything moving there'd be settings in the frontend to disable, for example, spin, tada, or shake (or just run them when hovering)

[aumetra]

And as for the parser.. I'm right now prototyping something with Logos, but especially for nested functions (e.g. $[tada $[shake $[3x hello!]]]) it seems a little verbose.. maybe I can do some things with winnow instead.

[aumetra]

Ugh, that nesting just makes things so much more complex than if I had a depth of 1