vscode-dts: Fix typedoc for WebviewPanel.dispose()

[hkleungai]

Sync the change from DefinitelyTyped/DefinitelyTyped#74140 back to this repo.

This change can be justified by the source code from src/vs/workbench/api/common/extHostWebviewPanels.ts.

public override dispose() {
  if (this.#isDisposed) {
    return;
  }

  this.#isDisposed = true;
  this.#onDidDispose.fire();

  this.#proxy.$disposeWebview(this.#handle);
  this.#webview.dispose();

  super.dispose();
}