DEV: transpile certain ES6 files without producing a module (#6460)

`.js.no-module.es6` files will be transpiled without producing a module.

import/export statements are deliberately not supported, so one would still need to use 
`const module = require('moduleName').default.`

lib/es6_module_transpiler/sprockets.rb

@@ -1,4 +1,4 @@
 require 'sprockets'
 
-Sprockets.register_mime_type 'application/ecmascript6', extensions: ['.es6', '.js.es6'], charset: :unicode
+Sprockets.register_mime_type 'application/ecmascript6', extensions: ['.es6', '.js.es6', '.js.no-module.es6'], charset: :unicode
 Sprockets.register_transformer 'application/ecmascript6', 'application/javascript', Tilt::ES6ModuleTranspilerTemplate

lib/es6_module_transpiler/tilt/es6_module_transpiler_template.rb

@@ -178,7 +178,7 @@ JS
 
       js_source = ::JSON.generate(source, quirks_mode: true)
 
-      if opts[:module_name]
+      if opts[:module_name] && transpile_into_module?
         filename = opts[:filename] || 'unknown'
         "Babel.transform(#{js_source}, { moduleId: '#{opts[:module_name]}', filename: '#{filename}', ast: false, presets: ['es2015'], plugins: [['transform-es2015-modules-amd', {noInterop: true}], 'transform-decorators-legacy', exports.WidgetHbsCompiler] }).code"
       else
@@ -188,6 +188,10 @@ JS
 
     private
 
+    def transpile_into_module?
+      file.nil? || file.exclude?('.no-module')
+    end
+
     def module_name(root_path, logical_path)
       path = nil