fix: dispose webviewPanel in InfoProvider.dispose (#514)

The InfoProvider opens the webviewPanel, so it should also call the
webviewPanel's dispose function when gettting disposed.

vscode-lean4/src/infoview.ts

@@ -481,6 +481,7 @@ export class InfoProvider implements Disposable {
         // active client is changing.
         this.clearNotificationHandlers()
         this.clearRpcSessions(null)
+        this.webviewPanel?.dispose()
         for (const s of this.clientSubscriptions) {
             s.dispose()
         }