Enable projects to opt-out of workspace management (#4565)
## Summary
You can now add `managed = false` under `[tool.uv]` in a
`pyproject.toml` to explicitly opt out of the project and workspace
APIs.
If a project sets `managed = false`, we will (1) _not_ discover it as a
workspace root, and (2) _not_ discover it as a workspace member (similar
to using `exclude` in the workspace parent).
Closes https://github.com/astral-sh/uv/issues/4551.
