Fix ify-loader test on Windows / Cygwin

On Windows and Cygwin the path to JS source files may contain
backslashes, forward slashes, or a mix. The regex used to test whether
or not to use `ify-loader` contains path separators; this change allows
either slash instead of requiring only forward slashes.

webpack.config.js

@@ -22,7 +22,7 @@ const base = {
                 }
             },
             {
-                test: /node_modules\/(linebreak|grapheme-breaker)\/.*\.js$/,
+                test: /node_modules[\\/](linebreak|grapheme-breaker)[\\/].*\.js$/,
                 loader: 'ify-loader'
             }
         ]