feat(editor): is nothing selected command (#10721)

### TL;DR
Added a new command to check if nothing is currently selected in the editor.

### What changed?
- Created new `isNothingSelectedCommand` to determine if there are no active selections

blocksuite/affine/shared/src/commands/selection/index.ts

@@ -7,4 +7,5 @@ export {
   getSelectionRectsCommand,
   type SelectionRect,
 } from './get-selection-rects';
-export { getTextSelectionCommand } from './get-text-selection';
+export { getTextSelectionCommand } from './get-text-selection.js';
+export { isNothingSelectedCommand } from './is-nothing-selected.js';

blocksuite/affine/shared/src/commands/selection/is-nothing-selected.ts

@@ -0,0 +1,28 @@
+import {
+  BlockSelection,
+  type Command,
+  TextSelection,
+} from '@blocksuite/block-std';
+
+import { ImageSelection } from '../../selection';
+
+export const isNothingSelectedCommand: Command<
+  {
+    currentTextSelection?: TextSelection;
+    currentImageSelections?: ImageSelection[];
+    currentBlockSelections?: BlockSelection[];
+  },
+  { isNothingSelected: boolean }
+> = (ctx, next) => {
+  const textSelection =
+    ctx.currentTextSelection || ctx.std.selection.find(TextSelection);
+  const imageSelections =
+    ctx.currentImageSelections || ctx.std.selection.filter(ImageSelection);
+  const blockSelections =
+    ctx.currentBlockSelections || ctx.std.selection.filter(BlockSelection);
+  const isNothingSelected =
+    !textSelection &&
+    imageSelections.length === 0 &&
+    blockSelections.length === 0;
+  next({ isNothingSelected });
+};