files: Fix event listener leak (#1504)

This would slowly leak memory and break games that need to detect the
escape button

extensions/files.js

@@ -57,7 +57,9 @@
         _resolve(text);
         Scratch.vm.renderer.removeOverlay(outer);
         Scratch.vm.runtime.off("PROJECT_STOP_ALL", handleProjectStopped);
-        document.body.removeEventListener("keydown", handleKeyDown);
+        document.body.removeEventListener("keydown", handleKeyDown, {
+          capture: true,
+        });
       };
 
       let isReadingFile = false;