PB-369: Integrate third party library to work with DOMDocument

 - add PhpGt DOM library

Author: cspruiell

app/code/Magento/PageBuilder/composer.json

@@ -22,6 +22,9 @@
         "magento/module-require-js": "*",
         "php": "~7.1.3||~7.2.0||~7.3.0"
     },
+    "require-dev": {
+        "phpgt/dom": "^2.1.6"
+    },
     "suggest": {
         "magento/module-review": "*"
     },