Add --app and --lib options to uv init

[zanieb]

Changes the uv init experience with a focus on working for more use-cases out of the box.

• Adds --app and --lib options to control the created project style
• Changes the default from a library with src/ and a build backend (--lib) to an application that is not packaged (--app)
• Hides the --virtual option and replaces it with --package and --no-package
• --no-package is not allowed with --lib right now, but it could be in the future once we understand a use-case
• Creates a runnable project
  • Applications have a hello.py file which you can run with uv run hello.py
  • Packaged applications, e.g., uv init --app --package create a package and script entrypoint, which you can run with uv run hello
  • Libraries provide a demo API function, e.g., uv run python -c "import name; print(name.hello())" — this is unchanged

Closes #6471

[zanieb]

Instead of hello.py I explored app/__main__.py and uv run python -m app. I'm not sure what's better, but we might want to go with the simplest possible option? Thoughts? Seems nice to have some namespacing? Maybe we can do something different depending on if the directory is empty or not?