chore: add isFunction utils and improve isObject

src/utils/deep-assign.ts

@@ -1,4 +1,4 @@
-import { isDef, isObj } from '.';
+import { isDef, isObject } from '.';
 import { ObjectIndex } from './types';
 
 const { hasOwnProperty } = Object.prototype;
@@ -10,11 +10,7 @@ function assignKey(to: ObjectIndex, from: ObjectIndex, key: string) {
     return;
   }
 
-  if (
-    !hasOwnProperty.call(to, key) ||
-    !isObj(val) ||
-    typeof val === 'function'
-  ) {
+  if (!hasOwnProperty.call(to, key) || !isObject(val)) {
     to[key] = val;
   } else {
     // eslint-disable-next-line no-use-before-define