js: Use inside variant of IIFE wrappers.

Prettier would do this anyway, but it’s separated out for a more
reviewable diff.  Generated by ESLint.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/js/blueslip.js

@@ -68,7 +68,7 @@ Logger.prototype = (function () {
     }
 
     return proto;
-}());
+})();
 
 const logger = new Logger();