vscode extension: make sure the "Slint LSP" output always exist

It is used to show the output of `debug()`.
User don't find this output if it doesn't exist before there is any
debug output. So make sure it always exist.

(Note: this is not for the browser version, since then we use
console.log instead of the "Slint LSP" output from stderr)

editors/vscode/src/extension.ts

@@ -141,6 +141,9 @@ function startClient(
 
     // Add setup common between native and wasm LSP to common.setup_client_handle!
     client.add_updater((cl) => {
+        // Just make sure that the output channel is always present.
+        cl?.outputChannel.append("");
+
         cl?.onNotification(common.serverStatus, (params: any) =>
             common.setServerStatus(params, statusBar),
         );