Add support for extensions / plugins

[kkoreilly]

This would be very useful in apps like gide. It is not clear how we would implement this; Go's plugin system is too rigid and unsupported, so we would have to use something like https://github.com/hashicorp/go-plugin or https://github.com/traefik/yaegi. We will probably not do this in time for v2.

[kkoreilly]

WASM / WASI is probably the best way to do this.

[rcoreilly]

sounds like a technical exploration issue that needs to happen in the context of a specific use-case -- could discuss that but at this point nothing concrete to discuss.