refactor(theme/view): drop lodash.assignIn (hexojs#3969)

lib/theme/view.js

@@ -1,10 +1,28 @@
 'use strict';
 
 const { dirname, extname, join } = require('path');
-const assignIn = require('lodash/assignIn');
 const yfm = require('hexo-front-matter');
 const Promise = require('bluebird');
 
+const assignIn = (...args) => {
+  const length = args.length;
+  const obj = args[0];
+
+  if (length < 2 || obj == null) return obj;
+  for (let index = 1; index < length; index++) {
+    const source = args[index];
+    const keys = [];
+    for (const key in source) keys.push(key);
+
+    const l = keys.length;
+    for (let i = 0; i < l; i++) {
+      const key = keys[i];
+      obj[key] = source[key];
+    }
+  }
+  return obj;
+};
+
 function View(path, data) {
   this.path = path;
   this.source = join(this._theme.base, 'layout', path);