Add support for uv init --script

[tfsingh]

This PR adds support for uv init --script, as defined in issue #7402 (started working on this before I saw jbvsmo's PR). Wanted to highlight a few decisions I made that differ from the existing PR:

1. --script takes a path, instead of a path/name. This potentially leads to a little ambiguity (I can certainly elaborate in the docs, lmk!), but strictly allowing uv init --script path/to/script.py felt a little more natural than allowing for uv init --script path/to --name script.py (which I also thought would prompt more questions for users, such as should the name include the .py extension?)
2. The request is processed immediately in the init method, sharing logic in resolving which python version to use with uv add --script. This made more sense to me — since scripts are meant to operate in isolation, they shouldn't consider the context of an encompassing package should one exist (I also think this decision makes the relative codepaths for scripts/packages easier to follow).
3. No readme — readme felt a little excessive for a script, but I can of course add it in!

@jbvsmo – hope it's okay I'm using the documentation you wrote!

[tfsingh]

Using try_exists here in line with a comment on the previous PR, but should we potentially use fs_err::tokio::metadata so it's async? (Although I don't suspect it's an expensive operation)