Add knot.toml schema (#15735)

## 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. ![Screen
Shot 2025-02-06 at 14 05 35
PM](https://github.com/user-attachments/assets/d7f3c2a9-2351-4226-9fc1-b91aa192a237)
* The property description mushes together the description of the
property and the value, which looks sort of ridiculous. ![Screen Shot
2025-02-06 at 14 04 40
PM](https://github.com/user-attachments/assets/8b72f04a-c62a-49b5-810f-7ddd472884d0)
* 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 ![Screen Shot 2025-02-06 at 14 08 19
PM](https://github.com/user-attachments/assets/78a7f849-ff4e-44ff-8317-708eaf02dc1f)


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

crates/ruff_db/Cargo.toml

@@ -30,6 +30,7 @@ glob = { workspace = true }
 ignore = { workspace = true, optional = true }
 matchit = { workspace = true }
 salsa = { workspace = true }
+schemars = { workspace = true, optional = true }
 serde = { workspace = true, optional = true }
 path-slash = { workspace = true }
 thiserror = { workspace = true }

crates/ruff_db/src/system/path.rs

@@ -471,7 +471,13 @@ impl ToOwned for SystemPath {
 /// The path is guaranteed to be valid UTF-8.
 #[repr(transparent)]
 #[derive(Eq, PartialEq, Clone, Hash, PartialOrd, Ord)]
-pub struct SystemPathBuf(Utf8PathBuf);
+#[cfg_attr(
+    feature = "serde",
+    derive(serde::Serialize, serde::Deserialize),
+    serde(transparent)
+)]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+pub struct SystemPathBuf(#[cfg_attr(feature = "schemars", schemars(with = "String"))] Utf8PathBuf);
 
 impl SystemPathBuf {
     pub fn new() -> Self {
@@ -658,27 +664,6 @@ impl ruff_cache::CacheKey for SystemPathBuf {
     }
 }
 
-#[cfg(feature = "serde")]
-impl serde::Serialize for SystemPath {
-    fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
-        self.0.serialize(serializer)
-    }
-}
-
-#[cfg(feature = "serde")]
-impl serde::Serialize for SystemPathBuf {
-    fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
-        self.0.serialize(serializer)
-    }
-}
-
-#[cfg(feature = "serde")]
-impl<'de> serde::Deserialize<'de> for SystemPathBuf {
-    fn deserialize<D: serde::Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
-        Utf8PathBuf::deserialize(deserializer).map(SystemPathBuf)
-    }
-}
-
 /// A slice of a virtual path on [`System`](super::System) (akin to [`str`]).
 #[repr(transparent)]
 pub struct SystemVirtualPath(str);