Use standalone Python builds when creating a virtual environment

[pfmoore]

When trying to reproduce an issue on a Python version that I don't have installed, it would be useful to be able to create a virtual environment for that version without needing to install a system copy of the Python version first. Other tools like hatch and PDM use the Python standalone builds project for this. Clearly I could download the appropriate interpreter, unpack it, and run uv venv --python /path/to/unpacked/build, but that would mean manually managing the unpacked version.

Being able to do something like uv venv --python pypy@3.10, and have uv download, unpack and cache the interpreter for me would be useful in this situation.

I should note that this isn't a common occurrence for me, so I'm not asking for this feature to be prioritised. The workarounds I currently have are perfectly fine. But this does feel to me like a natural extension to the current uv venv feature set.

[zanieb]

We'll support this.

[zanieb]

Closing in favor of #3061