[Build] Declare screenfull

...and shim such that it can be acquired in the normal AMD way.
Avoids use of global variable to satisfy JSHint.

main.js

@@ -44,6 +44,9 @@ requirejs.config({
         },
         "moment-duration-format": {
             "deps": [ "moment" ]
+        },
+        "screenfull": {
+            "exports": "screenfull"
         }
     }
 });

platform/commonUI/browse/src/windowing/FullscreenAction.js

@@ -25,7 +25,7 @@
  */
 define(
     ["screenfull"],
-    function () {
+    function (screenfull) {
 
         var ENTER_FULLSCREEN = "Enter full screen mode",
             EXIT_FULLSCREEN = "Exit full screen mode";

test-main.js

@@ -65,6 +65,9 @@ requirejs.config({
         },
         "moment-duration-format": {
             "deps": [ "moment" ]
+        },
+        "screenfull": {
+            "exports": "screenfull"
         }
     },