fix type incompatibility with array.sort()

mongoose/mongoose-tests.ts

@@ -446,7 +446,7 @@ mongooseArray.pull(5, 4, 'hi').$shift();
 mongooseArray.push([]).toFixed();
 mongooseArray.set(1, 'hi').$shift();
 mongooseArray.shift().toLowerCase();
-mongooseArray.sort(function (a: any, b: any) {
+mongooseArray.sort(function (a, b) {
   return a.length - b.length;
 }).unshift();
 mongooseArray.splice(4, 1).unshift();

mongoose/mongoose.d.ts

@@ -1141,7 +1141,7 @@ declare module "mongoose" {
          * potentially overwritting any changes that happen between when you retrieved the object
          * and when you save it.
          */
-        sort(compare?: Function): T[];
+        sort(compareFn?: (a: T, b: T) => number): T[];
 
         /**
          * Wraps Array#splice with proper change tracking and casting.