Add `--app` and `--lib` options to `uv init` (#6689)
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
