* 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.
