types(query): add $expr to RootQuerySelector

Fix #14841
Re: #14764

test/types/queries.test.ts

@@ -672,3 +672,9 @@ async function gh14545() {
   const myProjection = await M.findOne({}).select<SlimTest>({ prop: 1 }).exec();
   expectType<SlimTestDocument | null>(myProjection);
 }
+
+function gh14841() {
+  const filter: FilterQuery<{ owners: string[] }> = {
+    $expr: { $lt: [{ $size: "$owners" }, 10] },
+  };
+}

types/query.d.ts

@@ -116,6 +116,7 @@ declare module 'mongoose' {
     $where?: string | Function;
     /** @see https://www.mongodb.com/docs/manual/reference/operator/query/comment/#op._S_comment */
     $comment?: string;
+    $expr?: Record<string, any>;
     // we could not find a proper TypeScript generic to support nested queries e.g. 'user.friends.name'
     // this will mark all unrecognized properties as any (including nested queries) only if
     // they include a "." (to avoid generically allowing any unexpected keys)