[feat] Extension support

[unsecretised]

This adds support for extensions / plugins in rustcast

[wiiznokes]

I'm curious, how you plan to implement this ?

[unsecretised]

Similar to how zed does it, using WASM

[Mnem42]

Guess it'd probably need a rework of how apps are implemented

[unsecretised]

Ok i'll work on this and use a bunjs runtime, with html css and js to render the output