add basic implementation for attributes

packages/core/src/utils/getRenderedAttributes.ts

@@ -0,0 +1,17 @@
+import { Node } from 'prosemirror-model'
+import { ExtensionAttribute } from '../types'
+
+export default function getRenderedAttributes(node: Node, attributes: ExtensionAttribute[]) {
+  return attributes
+    .map(attribute => {
+      // TODO: fallback if renderHTML doesn’t exist
+      return attribute.attribute.renderHTML(node.attrs)
+    })
+    .reduce((accumulator, value) => {
+      // TODO: add support for "class" merge
+      return {
+        ...accumulator,
+        ...value,
+      }
+    }, {})
+}