Pluggable Markdown transformers

[brendenehlers]

Is your feature request related to a problem? Please describe.
Looking for a way to add custom Markdown transformers.

Referenced in this stale issue: #892

Describe the solution you'd like
I'd like to be able to define a custom Markdown transformer that is used like the magic-move or code block transformers, using the APIs already provided in this file:
https://github.com/slidevjs/slidev/blob/main/packages/slidev/node/vite/markdown.ts

Describe alternatives you've considered
The solution for !892 was to use custom Vite plugins, but that seems to be a bit overkill if the goal is just to implement a new feature in the Markdown slides.

I've also written a proof-of-concept in a fork, which shows that what I'm trying to do is possible, if the transformers are pluggable. Link here: brendenehlers@853f453

Please let me know if I missed an issue that already address this or if there's a page in the documentation on how to do this!

[KermanX]

Completed via #1767.