feat: Display a 'wait' cursor when opening trashcan (#6699)

No visible change for regular case.  But if there's thousands of blocks in the trash, the visual feedback that something's happening is better.  Otherwise the user clicks, and that click will cause some random block to be spawned out of the trash.

core/trashcan.ts

@@ -275,7 +275,13 @@ export class Trashcan extends DeleteArea implements IAutoHideable,
     const contents = this.contents_.map(function(string) {
       return JSON.parse(string);
     });
-    this.flyout?.show(contents);
+    // Trashcans with lots of blocks can take a second to render.
+    const blocklyStyle = this.workspace.getParentSvg().style;
+    blocklyStyle.cursor = 'wait';
+    setTimeout(() => {
+      this.flyout?.show(contents);
+      blocklyStyle.cursor = '';
+    }, 10);
     this.fireUiEvent_(true);
   }