add deep merge

packages/core/src/utilities/isObject.ts

@@ -0,0 +1,3 @@
+export default function isObject(item: any): boolean {
+  return (item && typeof item === 'object' && !Array.isArray(item))
+}

packages/core/src/utilities/mergeDeep.ts

@@ -0,0 +1,22 @@
+import { AnyObject } from '../types'
+import isObject from './isObject'
+
+export default function mergeDeep(target: AnyObject, source: AnyObject) {
+  const output = { ...target }
+
+  if (isObject(target) && isObject(source)) {
+    Object.keys(source).forEach(key => {
+      if (isObject(source[key])) {
+        if (!(key in target)) {
+          Object.assign(output, { [key]: source[key] })
+        } else {
+          output[key] = mergeDeep(target[key], source[key])
+        }
+      } else {
+        Object.assign(output, { [key]: source[key] })
+      }
+    })
+  }
+
+  return output
+}