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-db/src/assists.rs

@@ -29,7 +29,16 @@ pub struct Assist {
     /// cumbersome, especially if you want to embed an assist into another data
     /// structure, such as a diagnostic.
     pub source_change: Option<SourceChange>,
-    pub trigger_signature_help: bool,
+    /// The command to execute after the assist is applied.
+    pub command: Option<Command>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Command {
+    /// Show the parameter hints popup.
+    TriggerSignatureHelp,
+    /// Rename the just inserted item.
+    Rename,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]