cleanup: Use Array.includes() to check for element existence

We can use that newer method where we don't care about the actual position
of an element inside the array.

(Array.includes() and Array.indexOf() do behave differently in edge cases,
for example in the handling of NaN, but those don't matter to us)

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/152

js/misc/extensionUtils.js

@@ -31,7 +31,7 @@ function getCurrentExtension() {
     // Search for an occurrence of an extension stack frame
     // Start at 1 because 0 is the stack frame of this function
     for (let i = 1; i < stack.length; i++) {
-        if (stack[i].indexOf('/gnome-shell/extensions/') > -1) {
+        if (stack[i].includes('/gnome-shell/extensions/')) {
             extensionStackLine = stack[i];
             break;
         }