sc-el as submodule

Hey all,

We have a few supercollider users interested in maintaining the scel
project. They've requested moving it to a separate submodule like scvim,
which I think is a good idea. See this ticket for the conversation:

https://github.com/supercollider/supercollider/issues/3066

Since nilninull and widp don't have a contribution past in the
SuperCollider project, I was thinking we would create the submodule first
and hold off giving them admin access for a bit. I think that's only fair;
it doesn't seem like there's anything to worry about in terms of intentions
or collaboration.

Thoughts/opinions welcome.

Regards,
Brian