Add `--script` support to `uv export` for PEP 723 scripts (#10160)
## Summary
You can now run `uv export --script main.py` to show the dependency tree
for a given script. If a lockfile doesn't exist, it will create one.
Closes https://github.com/astral-sh/uv/issues/8609.
Closes https://github.com/astral-sh/uv/issues/9657.
