ruff server: Introduce the ruff.printDebugInformation command (#11831)

## Summary

Closes #11715.

Introduces a new command, `ruff.printDebugInformation`. This will print
useful information about the status of the server to `stderr`.

Right now, the information shown by this command includes:
* The path to the server executable
* The version of the executable
* The text encoding being used
* The number of open documents and workspaces
* A list of registered configuration files
* The capabilities of the client

## Test Plan

First, checkout and use [the corresponding `ruff-vscode`
PR](https://github.com/astral-sh/ruff-vscode/pull/495).

Running the `Print debug information` command in VS Code should show
something like the following in the Output channel:

<img width="991" alt="Screenshot 2024-06-11 at 11 41 46 AM"
src="ab93c009-bb7b-4291-b057-d44fdc6f9f86">

crates/ruff_server/src/session.rs

@@ -145,6 +145,18 @@ impl Session {
         Ok(())
     }
 
+    pub(crate) fn num_documents(&self) -> usize {
+        self.index.num_documents()
+    }
+
+    pub(crate) fn num_workspaces(&self) -> usize {
+        self.index.num_workspaces()
+    }
+
+    pub(crate) fn list_config_files(&self) -> Vec<&std::path::Path> {
+        self.index.list_config_files()
+    }
+
     pub(crate) fn resolved_client_capabilities(&self) -> &ResolvedClientCapabilities {
         &self.resolved_client_capabilities
     }