fix linting problems and add this type to default function

Author: pshaddel

test/types/schema.test.ts

@@ -1223,5 +1223,10 @@ async function gh13797() {
   interface IUser {
     name: string;
   }
-  const schema2 = new Schema<IUser>({ name: { type: String, required: function () { expectType<IUser>(this); return true } } });
+  new Schema<IUser>({ name: { type: String, required: function() {
+    expectType<IUser>(this); return true;
+  } } });
+  new Schema<IUser>({ name: { type: String, default: function() {
+    expectType<IUser>(this); return '';
+  } } });
 }

types/schematypes.d.ts

@@ -80,7 +80,7 @@ declare module 'mongoose' {
      * The default value for this path. If a function, Mongoose executes the function
      * and uses the return value as the default.
      */
-    default?: DefaultType<T> | ((this: any, doc: any) => DefaultType<T>) | null;
+    default?: DefaultType<T> | ((this: EnforcedDocType, doc: any) => DefaultType<T>) | null;
 
     /**
      * The model that `populate()` should use if populating this path.