fix(types): add Model#addScope overload

fix(types): add Model#addScope overload

Adds the overloaded signature for `Model#addScope`, and some JSDocs for
the method

types/lib/model.d.ts

@@ -1709,7 +1709,16 @@ export abstract class Model<T = any, T2 = any> extends Hooks {
     options?: string | ScopeOptions | (string | ScopeOptions)[] | WhereAttributeHash
   ): M;
 
+  /**
+   * Add a new scope to the model
+   *
+   * This is especially useful for adding scopes with includes, when the model you want to
+   * include is not available at the time this model is defined. By default this will throw an
+   * error if a scope with that name already exists. Pass `override: true` in the options
+   * object to silence this error.
+   */
   public static addScope(name: string, scope: FindOptions, options?: AddScopeOptions): void;
+  public static addScope(name: string, scope: (...args: any[]) => FindOptions, options?: AddScopeOptions): void;
 
   /**
    * Search for multiple instances.

types/test/models/User.ts

@@ -6,7 +6,8 @@ import {
   DataTypes,
   FindOptions,
   Model,
-  ModelCtor
+  ModelCtor,
+  Op
 } from 'sequelize';
 import { sequelize } from '../connection';
 
@@ -74,6 +75,22 @@ User.addHook('beforeFind', 'test', (options: FindOptions) => {
   return undefined;
 });
 
+// Model#addScope
+User.addScope('withoutFirstName', {
+  where: {
+    firstName: {
+      [Op.is]: null,
+    },
+  },
+});
+
+User.addScope(
+  'withFirstName',
+  (firstName: string) => ({
+    where: { firstName },
+  }),
+);
+
 // associate
 // it is important to import _after_ the model above is already exported so the circular reference works.
 import { UserGroup } from './UserGroup';