Add user_configuration_directory to System (#16020)

## Summary

This PR adds a new `user_configuration_directory` method to `System`. We
need it to resolve where to lookup a user-level `knot.toml`
configuration file.
The method belongs to `System` because not all platforms have a
convention of where to store such configuration files (e.g. wasm).


I refactored `TestSystem` to be a simple wrapper around an `Arc<dyn
System...>` and use the `System.as_any` method instead to cast it down
to an `InMemory` system. I also removed some `System` specific methods
from `InMemoryFileSystem`, they don't belong there.

This PR removes the `os` feature as a default feature from `ruff_db`.
Most crates depending on `ruff_db` don't need it because they only
depend on `System` or only depend on `os` for testing. This was
necessary to fix a compile error with `red_knot_wasm`

## Test Plan

I'll make use of the method in my next PR. So I guess we won't know if
it works before then but I copied the code from Ruff/uv, so I have high
confidence that it is correct.

`cargo test`

crates/red_knot_python_semantic/Cargo.toml

@@ -44,7 +44,7 @@ test-case = { workspace = true }
 memchr = { workspace = true }
 
 [dev-dependencies]
-ruff_db = { workspace = true, features = ["os", "testing"] }
+ruff_db = { workspace = true, features = ["testing"] }
 ruff_python_parser = { workspace = true }
 red_knot_test = { workspace = true }
 red_knot_vendored = { workspace = true }