Gh-4044: Enabling ES6 in javascript client (#5593)

* gh-4044: Added useES6 as an option for javascript templates

* gh-4044: Enabled ES6 in `javascript/api.mustache`

* gh-4044: Enabled ES6 in `javascript/ApiClient.mustache`

* gh-4044: Enabled ES6 in `javascript/enumClass.mustache`

* gh-4044: Added useES6 cli option to `javascript` clients and updated the test cases

* gh-4044: Enabled ES6 in `javascript/index.mustache`

* gh-4044: Enabled ES6 in `javascript` model templates
* `javascript/model.mustache`
* `javascript/partial_model_generic.mustache`
* `javascript/partial_model_enum_class.mustache`
* `javascript/partial_model_inner_enum.mustache`

* gh-4044: Separated `javascript-es6` templates to another folder

* gh-4044: Updated `javascript-es6/index.mustache`

* gh-4044: Enabled ES6 in `javascript-es6/api_doc.mustache`

* gh-4044: Added required dependencies for ES6

* gh-4044: Updated Supportig files for ES6 and non ES6

* gh-4044: Added test scripts to verify `javascript` useEs6 option

* gh-4044: Commented `javascript-es6` scripts due to the permission issues.

samples/client/petstore/javascript-es6/docs/Order.md

@@ -0,0 +1,26 @@
+# SwaggerPetstore.Order
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**id** | **Number** |  | [optional] 
+**petId** | **Number** |  | [optional] 
+**quantity** | **Number** |  | [optional] 
+**shipDate** | **Date** |  | [optional] 
+**status** | **String** | Order Status | [optional] 
+**complete** | **Boolean** |  | [optional] [default to false]
+
+
+<a name="StatusEnum"></a>
+## Enum: StatusEnum
+
+
+* `placed` (value: `"placed"`)
+
+* `approved` (value: `"approved"`)
+
+* `delivered` (value: `"delivered"`)
+
+
+
+