uv is a fast alternative to pip and venv.

Building venvs with pip is a significant bottleneck in thx, as pip is slow, single-threaded, and caches only metadata. uv caches downloads as well, installs in parallel, and can rebuild venvs almost instantly.

Additionally uv has the ability to provision Python interpreters, which is valuable in a multi-version tool like thx.

We should add the option to use uv in order to take advantage of these capabilities.

When using uv we should allow uv configuration to dictate whether interpreters are downloaded or located on $PATH.