## Summary
This PR follows Cargo's strategy for merging configuration, albeit in a
more limited way (we don't support as many configuration locations).
Specifically, we merge the user configuration with the workspace
configuration if both are present. The workspace configuration has
priority, such that we take values from the workspace configuration and
ignore those in the user configuration if both are specified for a given
setting -- with the exception of arrays and maps, which are
concatenated.
For now, if a user provides a configuration file with `--config-file`,
we _don't_ merge in the user settings.
See:
https://doc.rust-lang.org/cargo/reference/config.html#hierarchical-structure.
Closes#3420.
## Summary
This PR adds basic struct definitions along with a "workspace" concept
for discovering settings. (The "workspace" terminology is used to match
Ruff; I did not invent it.)
A few notes:
- We discover any `pyproject.toml` or `uv.toml` file in any parent
directory of the current working directory. (We could adjust this to
look at the directories of the input files.)
- We don't actually do anything with the configuration yet; but those
PRs are large and I want this to be reviewed in isolation.
