WYSWIYG code blocks: copied head styles into shadow root

Currently only link-based styles are made available in the shadow root
code editor environment, this adds normal styles to apply any user-added
via custom head content.

Fixes #4228

Author: ssddanbrown

resources/js/wysiwyg/plugin-codeeditor.js

@@ -53,8 +53,8 @@ function defineCodeBlockCustomElement(editor) {
             super();
             this.attachShadow({mode: 'open'});
 
-            const stylesToCopy = document.querySelectorAll('link[rel="stylesheet"]:not([media="print"])');
-            const copiedStyles = Array.from(stylesToCopy).map(styleEl => styleEl.cloneNode(false));
+            const stylesToCopy = document.head.querySelectorAll('link[rel="stylesheet"]:not([media="print"]),style');
+            const copiedStyles = Array.from(stylesToCopy).map(styleEl => styleEl.cloneNode(true));
 
             const cmContainer = document.createElement('div');
             cmContainer.style.pointerEvents = 'none';