browser-support: Add codepointat polyfill to `common` entry point.

Adds string.prototype.codepointat which was added as a polyfill
earlier to the project but was not added to `common` entry point.

tools/webpack.assets.json

@@ -5,6 +5,7 @@
     ],
     "common": [
                "string.prototype.startswith",
+               "string.prototype.codepointat",
                "./node_modules/jquery/dist/jquery.js",
                "./node_modules/underscore/underscore.js",
                "./static/js/blueslip.js",