Rejiggering the branches

As we move forward, and try to add some new facilities to ASDF, this seems like a good time to revise the branching structure that we use.

In particular, I would like to add a `stable` branch that will permit continuation of the 3.3 release series, the need for which is illustrated by Mark E's recent merge request.

If we are going to make that change, it seems like a good time to rename the `master` branch to either `main` or `dev`. Any preferences?

I'd like to have a quick discussion about this over the week, with an eye to getting a solution in place by Friday.

Thanks for your thoughts,

R