ruff server: Support the usage of tildes and environment variables in logFile (#11945)

## Summary

Fixes #11911.

`shellexpand` is now used on `logFile` to expand the file path, allowing
the usage of `~` and environment variables.

## Test Plan

1. Set `logFile` in either Neovim or Helix to a file path that needs
expansion, like `~/.config/helix/ruff_logs.txt`.
2. Ensure that `RUFF_TRACE` is set to `messages` or `verbose`
3. Open a Python file in Neovim/Helix
4. Confirm that a file at the path specified was created, with the
expected logs.

crates/ruff_server/src/session/settings.rs

@@ -83,6 +83,7 @@ pub(crate) struct ClientSettings {
 #[serde(rename_all = "camelCase")]
 pub(crate) struct TracingSettings {
     pub(crate) log_level: Option<crate::trace::LogLevel>,
+    /// Path to the log file - tildes and environment variables are supported.
     pub(crate) log_file: Option<PathBuf>,
 }