util: Add classExtends function

Javascript doesn't have a builtin operator for checking if one class is
a subclass of another (ie. instanceof needs an instance, not a class).

This commit adds a new function, classExtends, that loops through the
class inheritance hierarchy to find if a relationship exists.

js/misc/util.js

@@ -279,6 +279,29 @@ export function lerp(start, end, progress) {
     return start + progress * (end - start);
 }
 
+/**
+ * @typedef {Function} Class
+ * @description A class
+ *
+ * Note: A class (as defined by the ES6 class keyword)
+ */
+
+/**
+ * @param {Class} subClass
+ * @param {Class} superClass
+ * @returns {boolean}
+ *
+ * Returns whether `subClass` is a subclass of `superClass` or not
+ */
+export function classExtends(subClass, superClass) {
+    for (let prototype = subClass?.prototype; prototype; prototype = Object.getPrototypeOf(prototype)) {
+        if (prototype === superClass?.prototype)
+            return true;
+    }
+
+    return false;
+}
+
 /**
  * _GNOMEversionToNumber:
  *