This introduces a dedicated package for rbext, the Review Board

extension command line tool. This is a straight port of what's in

the Review Board 8 tree, but with modern typing and code cleanup

integrated with it, and deprecations removed.
The plan is to have a standalone tool not dependent on any given version

of Review Board that can be used not only to create and test packages,

but also to build packages to distribute.
To accomplish building packages, an upcoming change will introduce a

Python build backend, rbext.build.backend, built on top of

buildthings. This will handle a pre-build phase involving asset

generation and static media infrastructure setup, and then perform a

standard python -m build.
For now, everything that's in here is more or less what was in

reviewboard/cmdline/rbext.py, but split up and cleaned up. This will

be the foundation for future work.

Tested creating a tree and then running unit tests on it.