Create type definition for 'koa-json-body' (#4056)

* Create type definition for 'koa-json-body'

* Follow lint

src/@types/koa-json-body.d.ts

@@ -0,0 +1,15 @@
+declare module 'koa-json-body' {
+	import { Middleware } from 'koa';
+
+	interface IKoaJsonBodyOptions {
+		strict: boolean;
+		limit: string;
+		fallback: boolean;
+	}
+
+	function koaJsonBody(opt?: IKoaJsonBodyOptions): Middleware;
+
+	namespace koaJsonBody {} // Hack
+
+	export = koaJsonBody;
+}