Trigger VSCode to rename after extract variable assist is applied

When the user applies the "Extract Variable" assist, the cursor is
positioned at the newly inserted variable. This commit adds a command
to the assist that triggers the rename action in VSCode. This way, the
user can quickly rename the variable after applying the assist.

Fixes part of: #17579

crates/ide-diagnostics/src/lib.rs

@@ -613,7 +613,7 @@ fn unresolved_fix(id: &'static str, label: &str, target: TextRange) -> Assist {
         group: None,
         target,
         source_change: None,
-        trigger_signature_help: false,
+        command: None,
     }
 }