feat(editor): content block getter command (#10720)

### TL;DR
Added new commands to retrieve the first and last content blocks in a document.

### What changed?
- Created `getFirstContentBlockCommand` to find the first content block in a document
- Created `getLastContentBlockCommand` to find the last content block in a document
- Added `getFirstNoteBlock` utility function to find the first note block in a document

blocksuite/affine/shared/src/utils/model/getter.ts

@@ -57,3 +57,19 @@ export function getLastNoteBlock(doc: Store) {
   }
   return note;
 }
+
+export function getFirstNoteBlock(doc: Store) {
+  let note: NoteBlockModel | null = null;
+  if (!doc.root) return null;
+  const { children } = doc.root;
+  for (const child of children) {
+    if (
+      matchModels(child, [NoteBlockModel]) &&
+      child.displayMode !== NoteDisplayMode.EdgelessOnly
+    ) {
+      note = child as NoteBlockModel;
+      break;
+    }
+  }
+  return note;
+}