refactor: put back Formatter grid argument as optional

Author: ghiscoding

src/app/modules/angular-slickgrid/models/formatter.interface.ts

@@ -1,4 +1,4 @@
 import { Column } from './column.interface';
 import { FormatterResultObject } from './formatterResultObject.interface';
 
-export declare type Formatter = (row: number, cell: number, value: any, columnDef: Column, dataContext: any, grid: any) => string | FormatterResultObject;
+export declare type Formatter = (row: number, cell: number, value: any, columnDef: Column, dataContext: any, grid?: any) => string | FormatterResultObject;