Extend venv export to standalone scripts as well

[manzt]

Allows for passing a script with inline script metadata to juv venv.

uv init --script foo.py
uv add --script foo.py attrs
juv venv --from=foo.py
# Using CPython 3.13.0
# Creating virtual environment at: .venv
# Activate with: source .venv/bin/activate
# Using Python 3.13.0 environment at: .venv
# Resolved 1 package in 0.62ms
# Installed 1 package in 1ms
# + attrs==25.1.0

Useful for quickly creating a .venv for a standalone script, which can be
used by other tools like text editors or IDEs.