fix: deprecate the extend() function (#7944)

This function will be removed in Video.js 8.0

src/js/extend.js

@@ -4,12 +4,16 @@
  */
 
 import _inherits from '@babel/runtime/helpers/inherits';
+import log from './utils/log.js';
+
+let hasLogged = false;
 
 /**
  * Used to subclass an existing class by emulating ES subclassing using the
  * `extends` keyword.
  *
  * @function
+ * @deprecated
  * @example
  * var MyComponent = videojs.extend(videojs.getComponent('Component'), {
  *   myCustomMethod: function() {
@@ -27,6 +31,15 @@ import _inherits from '@babel/runtime/helpers/inherits';
  *           The new class with subClassMethods that inherited superClass.
  */
 const extend = function(superClass, subClassMethods = {}) {
+
+  // Log a warning the first time extend is called to note that it is deprecated
+  // It was previously deprecated in our documentation (guides, specifically),
+  // but was never formally deprecated in code.
+  if (!hasLogged) {
+    log.warn('videojs.extend is deprecated as of Video.js 7.22.0 and will be removed in Video.js 8.0.0');
+    hasLogged = true;
+  }
+
   let subClass = function() {
     superClass.apply(this, arguments);
   };