Add a uv-python shim executable

[zanieb]

Adds a minimal uv-python binary to our distributions, which uses uv python find to determine which Python executable to use then invokes it with all the arguments.

Some notes:

• We call this uv-python instead of python so we don't replace python on the PATH when uv is installed.
• Respects virtual environments, but we might want to skip virtual environments by default? We have a flag to opt-out per invocation, at least.
• We copy (or link) this into the user's bin as python during uv python install. We'll probably want more ways to manage the shim?
• We add an environment variable to Interpreter::query calls to avoid recursive queries once a shim is installed
• Adds --shim / --no-shim flags to uv python install. By default, we only install a shim if a CPython variant is installed and preview is enabled.

Supports a few options, which can be used together:

• Version requests e.g. uv-python +3.12 ....
• Only use a managed interpreter e.g. uv-python +managed ....
• Ignore virtual environments e.g. uv-python +system ....
• Enable verbose output e.g., uv-python +v ...

Unlike uvx, we use which to find the uv binary instead of only looking next to the file. This is because we need this to be copyable into the user's path. Unfortunately this makes it hard for us to guarantee that we are finding the correct version of uv if multiple are installed, but that seems like an edge-case. We could add some sort of UV_REQUIRED_VERSION variable or something that tells the uv binary to bail if it's not the right version. We prefer a binary that we find next to uv-python which could help alleviate this — I'll need to look at what happens with symlinks and such.

An alternative implementation is something like uvx where we add a uv python run command and then create aliases to it. That route is powerful because we can have advanced behaviors and rely on other crates without increasing the size of our installation. However, the constraints of this approach may be a good thing.

Unfortunately we need to invoke uv to find the Python interpreter to use and uv queries Python interpreters so this probably adds some significant overhead to a python invocation. I'll benchmark this before moving out of draft.