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/ruff_db/src/system.rs

@@ -7,7 +7,7 @@ use std::error::Error;
 use std::fmt::Debug;
 use std::path::{Path, PathBuf};
 use std::{fmt, io};
-pub use test::{DbWithTestSystem, TestSystem};
+pub use test::{DbWithTestSystem, InMemorySystem, TestSystem};
 use walk_directory::WalkDirectoryBuilder;
 
 use crate::file_revision::FileRevision;
@@ -99,6 +99,11 @@ pub trait System: Debug {
     /// Returns the current working directory
     fn current_directory(&self) -> &SystemPath;
 
+    /// Returns the directory path where user configurations are stored.
+    ///
+    /// Returns `None` if no such convention exists for the system.
+    fn user_config_directory(&self) -> Option<SystemPathBuf>;
+
     /// Iterate over the contents of the directory at `path`.
     ///
     /// The returned iterator must have the following properties: