Ensure that all import and require statements, in the entire code-base, have a .js file extension

In order to eventually get rid of SystemJS and start using native `import`s instead, we'll need to provide "complete" file identifiers since otherwise there'll be MIME type errors when attempting to use `import`.

src/display/display_utils.js

@@ -23,7 +23,7 @@ import {
   stringToBytes,
   Util,
   warn,
-} from "../shared/util";
+} from "../shared/util.js";
 
 const DEFAULT_LINK_REL = "noopener noreferrer nofollow";
 const SVG_NS = "http://www.w3.org/2000/svg";