## Summary
Adds a JSON schema generation step for Red Knot. This PR doesn't yet add
a publishing step because it's still a bit early for that
## Test plan
I tested the schema in Zed, VS Code and PyCharm:
* PyCharm: You have to manually add a schema mapping (settings JSON
Schema Mappings)
* Zed and VS code support the inline schema specification
```toml
#:schema /Users/micha/astral/ruff/knot.schema.json
[environment]
extra-paths = []
[rules]
call-possibly-unbound-method = "error"
unknown-rule = "error"
# duplicate-base = "error"
```
```json
{
"$schema": "file:///Users/micha/astral/ruff/knot.schema.json",
"environment": {
"python-version": "3.13",
"python-platform": "linux2"
},
"rules": {
"unknown-rule": "error"
}
}
```
https://github.com/user-attachments/assets/a18fcd96-7cbe-4110-985b-9f1935584411
The Schema overall works but all editors have their own quirks:
* PyCharm: Hovering a name always shows the section description instead
of the description of the specific setting. But it's the same for other
settings in `pyproject.toml` files 🤷
* VS Code (JSON): Using the generated schema in a JSON file gives
exactly the experience I want
* VS Code (TOML):
* Properties with multiple possible values are repeated during
auto-completion without giving any hint how they're different. 
* The property description mushes together the description of the
property and the value, which looks sort of ridiculous. 
* Autocompletion and documentation hovering works (except the
limitations mentioned above)
* Zed:
* Very similar to VS Code with the exception that it uses the
description attribute to distinguish settings with multiple possible
values 
I don't think there's much we can do here other than hope (or help)
editors improve their auto completion. The same short comings also apply
to ruff, so this isn't something new. For now, I think this is good
enough
## Summary
This PR adds support for configuring Red Knot in the `tool.knot` section
of the project's
`pyproject.toml` section. Options specified on the CLI precede the
options in the configuration file.
This PR only supports the `environment` and the `src.root` options for
now.
Other options will be added as separate PRs.
There are also a few concerns that I intentionally ignored as part of
this PR:
* Handling of relative paths: We need to anchor paths relative to the
current working directory (CLI), or the project (`pyproject.toml` or
`knot.toml`)
* Tracking the source of a value. Diagnostics would benefit from knowing
from which configuration a value comes so that we can point the user to
the right configuration file (or CLI) if the configuration is invalid.
* Schema generation and there's a lot more; see
https://github.com/astral-sh/ruff/issues/15491
This PR changes the default for first party codes: Our existing default
was to only add the project root. Now, Red Knot adds the project root
and `src` (if such a directory exists).
Theoretically, we'd have to add a file watcher event that changes the
first-party search paths if a user later creates a `src` directory. I
think this is pretty uncommon, which is why I ignored the complexity for
now but I can be persuaded to handle it if it's considered important.
Part of https://github.com/astral-sh/ruff/issues/15491
## Test Plan
Existing tests, new file watching test demonstrating that changing the
python version and platform is correctly reflected.
