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/EnumTest.md

@@ -0,0 +1,46 @@
+# SwaggerPetstore.EnumTest
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**enumString** | **String** |  | [optional] 
+**enumInteger** | **Number** |  | [optional] 
+**enumNumber** | **Number** |  | [optional] 
+**outerEnum** | [**OuterEnum**](OuterEnum.md) |  | [optional] 
+
+
+<a name="EnumStringEnum"></a>
+## Enum: EnumStringEnum
+
+
+* `UPPER` (value: `"UPPER"`)
+
+* `lower` (value: `"lower"`)
+
+* `empty` (value: `""`)
+
+
+
+
+<a name="EnumIntegerEnum"></a>
+## Enum: EnumIntegerEnum
+
+
+* `1` (value: `1`)
+
+* `-1` (value: `-1`)
+
+
+
+
+<a name="EnumNumberEnum"></a>
+## Enum: EnumNumberEnum
+
+
+* `1.1` (value: `1.1`)
+
+* `-1.2` (value: `-1.2`)
+
+
+
+