fix(types): add string to Includeable (#11003)

This is allowable for Aliases, according to docs and lack of errors when attempting.
options.include: http://docs.sequelizejs.com/class/lib/model.js~Model.html#static-method-findAll

types/lib/model.d.ts

@@ -376,7 +376,7 @@ export interface IncludeThroughOptions extends Filterable, Projectable {}
 /**
  * Options for eager-loading associated models, also allowing for all associations to be loaded at once
  */
-export type Includeable = typeof Model | Association | IncludeOptions | { all: true };
+export type Includeable = typeof Model | Association | IncludeOptions | { all: true } | string;
 
 /**
  * Complex include options

types/test/model.ts

@@ -23,6 +23,10 @@ MyModel.findOne({
   ]
 });
 
+MyModel.hasOne(OtherModel, { as: 'OtherModelAlias' });
+
+MyModel.findOne({ include: ['OtherModelAlias'] });
+
 const sequelize = new Sequelize('mysql://user:user@localhost:3306/mydb');
 
 MyModel.init({}, {