`bst shell` could use splitting into artifact/source commands

Background

While writing #1151 (closed) I realized that bst shell would make more sense if it was split into bst artifact shell and bst source shell, where bst source shell would take on the jobs of the former bst shell -b.

I think this is a worthwhile thought, because bst shell -b/--build has confused others in the past, and makes the concept of build trees more complex, since it only applies to one kind of shell.

Task description

bst shell -b should be renamed to bst source shell, and bst shell should be renamed to bst artifact shell.

This probably warrants some more discussion, since it's deprecating another major command.

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