Add a [ufl] install option to install Basix and UFL

[mscroggs]

Now that basix.ufl is an intended way to create UFL elements, it might be useful to be able to run pip install git+http://github.com/FEniCS/basix[ufl] to install UFL with Basix.

[garth-wells]

This will another line that we'll need to update at each release - is it worth it?

[mscroggs]

This will another line that we'll need to update at each release - is it worth it?

I'm not sure. Currently, just installing it with optional is fine I guess as that only has numba on top of ufl. Maybe worth revisiting in future if we ever add a lot more stuff to optional