feat: Add will-resize event to TopLevelWindow (#13649)

The new `will-resize` event can be used to prevent the resize from
happening before the native window is actually resized. This is in
contrast to the existing the `resize` event, which is sent *after* the
native window has already been resized. For apps with e.g. custom window
snapping logic, the `resize` event is not sufficient because it will
result in flickering between the dragged size and the snapped size.

`will-resize` is only emitted on macOS and Windows.

atom/browser/api/atom_api_top_level_window.cc

@@ -194,6 +194,13 @@ void TopLevelWindow::OnWindowRestore() {
   Emit("restore");
 }
 
+void TopLevelWindow::OnWindowWillResize(const gfx::Rect& new_bounds,
+                                        bool* prevent_default) {
+  if (Emit("will-resize", new_bounds)) {
+    *prevent_default = true;
+  }
+}
+
 void TopLevelWindow::OnWindowResize() {
   Emit("resize");
 }