Add `--no-workspace` and `--no-project` in lieu of `--isolated` (#5465)
## Summary
Right now, `--isolated` is read from `uv run` and `uv init` to avoid
discovering the current workspace (or project). This PR moves that
behavior to a dedicated `--no-workspace` flag for `uv init`, and
`--no-project` for `uv run`. They could use the same flag, but
`--no-project` feels confusing for `uv init`, and `--no-workspace` seems
confusing for `uv run` (especially so once you read the documentation,
where we refer to the thing you're omitting as the project).
Closes https://github.com/astral-sh/uv/issues/5429.