move extension helper functions to separate files

packages/core/src/utils/getMarksFromExtensions.ts

@@ -0,0 +1,10 @@
+import Mark from '../Mark'
+import collect from 'collect.js'
+import { Extensions } from '../types'
+
+export default function getMarksFromExtensions(extensions: Extensions): any {
+  return collect(extensions)
+    .where('extensionType', 'mark')
+    .mapWithKeys((extension: Mark) => [extension.name, extension.schema()])
+    .all()
+}