Remove var in Libraries/vendor/core/merge.js (#22108)

Summary:
Replaces the keywords var with const in Libraries/merge.js

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[GENERAL] [ENHANCEMENT] [Libraries/core] - remove var
Pull Request resolved: https://github.com/facebook/react-native/pull/22108

Differential Revision: D12919282

Pulled By: TheSavior

fbshipit-source-id: e3c8569c47c95ad3d1a061917c90185c2694c78f

Libraries/vendor/core/merge.js

@@ -7,7 +7,7 @@
 
 "use strict";
 
-var mergeInto = require('mergeInto');
+const mergeInto = require('mergeInto');
 
 /**
  * Shallow merges two structures into a return value, without mutating either.
@@ -16,8 +16,8 @@ var mergeInto = require('mergeInto');
  * @param {?object} two Optional object with properties to merge from.
  * @return {object} The shallow extension of one by two.
  */
-var merge = function(one, two) {
-  var result = {};
+const merge = function(one, two) {
+  const result = {};
   mergeInto(result, one);
   mergeInto(result, two);
   return result;