Implement `uv init` (#4791)
## Summary
Implements the `uv init` command, which initializes a project
(`pyproject.toml`, `README.md`, `src/__init__.py`) in the current
directory, or in the given path. `uv init` also does workspace
discovery.
Resolves https://github.com/astral-sh/uv/issues/1360.
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Co-authored-by: Zanie Blue <contact@zanie.dev>
