Support overriding the builtin list

[abathur]

I don't have any intrinsic objection to supporting shells other than bash, but I have very little interest in significantly complicating resholved to do so (especially in the short run.)

That said, I think the short-term Goldilocks change here is to support completely overriding the builtin list, which should enable resholved to resolve a fair fraction of the script base of a large number of traditional shells.

Since this seems likely to be a common request, I'm interested in finding space in the API for it now.