perf: lazy doc provider factory (#3330)

Co-authored-by: Alex Yang <himself65@outlook.com>

packages/y-provider/src/utils.ts

@@ -0,0 +1,14 @@
+import type { Doc } from 'yjs';
+
+export function getDoc(doc: Doc, guid: string): Doc | undefined {
+  if (doc.guid === guid) {
+    return doc;
+  }
+  for (const subdoc of doc.subdocs) {
+    const found = getDoc(subdoc, guid);
+    if (found) {
+      return found;
+    }
+  }
+  return undefined;
+}