Add support for uv init --script (#7565)

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!

---------

Co-authored-by: João Bernardo Oliveira <jbvsmo@gmail.com>

docs/guides/scripts.md

@@ -127,11 +127,20 @@ Multiple dependencies can be requested by repeating with `--with` option.
 Note that if `uv run` is used in a _project_, these dependencies will be included _in addition_ to
 the project's dependencies. To opt-out of this behavior, use the `--no-project` flag.
 
-## Declaring script dependencies
+## Creating a Python script
 
 Python recently added a standard format for
 [inline script metadata](https://packaging.python.org/en/latest/specifications/inline-script-metadata/#inline-script-metadata).
-This allows the dependencies for a script to be declared in the script itself.
+It allows for selecting Python versions and defining dependencies. Use `uv init --script` to
+initialize scripts with the inline metadata:
+
+```console
+$ uv init --script example.py --python 3.12
+```
+
+## Declaring script dependencies
+
+The inline metadata format allows the dependencies for a script to be declared in the script itself.
 
 uv supports adding and updating inline script metadata for you. Use `uv add --script` to declare the
 dependencies for the script: