Fix handlebars language loader

Handlebars shares a worker with html in the same way that
javascript/typescript and css/less/scss share workers. However, the
handlebars/html association was missing from the createLoaderRules
function. This was causing a bad network request when the language was
set to handlebars in the monaco-editor due to trying to load an
undefined web worker.

This change associates the handlebars and html workerPaths to prevent
these issues.

index.js

@@ -83,6 +83,11 @@ function createLoaderRules(languages, features, workers, outputPath, publicPath)
     workerPaths['scss'] = workerPaths['css'];
   }
 
+  if (workerPaths['html']) {
+    // handlebars and html share the same worker
+    workerPaths['handlebars'] = workerPaths['html'];
+  }
+
   const globals = {
     'MonacoEnvironment': `(function (paths) {
       function stripTrailingSlash(str) {