switch to BaseX

[bansp]

It's funny, @margaretha , that we haven't had an issue for that, yet :-)

Here's an idea: when we switch, let us leave the master branch as it is, and use 'main' as the branch for Bx development. We'll reset the defaults to that branch. And the master is going to stay locked, receiving only potential recommendation updates (redundantly until end-point switch), and then it's just going to, well, stay there, as part of the project history, while we keep working with dev, formats, and main.

[bansp]

Upon reflection, I still suggest that we switch to using 'main' as our main branch, but let us rename the old 'master' to 'exist' -- firstly, for self-documenting and secondly, so that we can keep doing git chec<TAB> m<TAB> :-)