add basic heading extension

docs/src/demos/HandleExtensions/index.vue

@@ -32,6 +32,7 @@ import Bold from '@tiptap/extension-bold'
 import Italic from '@tiptap/extension-italic'
 import Code from '@tiptap/extension-code'
 import CodeBlock from '@tiptap/extension-codeblock'
+import Heading from '@tiptap/extension-heading'
 
 export default {
   components: {
@@ -46,7 +47,7 @@ export default {
 
   mounted() {
     this.editor = new Editor({
-      content: '<p>This editor is based on Prosemirror, fully extendable and renderless. You can easily add custom nodes as Vue components.</p>',
+      content: '<h2>Hey there!</h2><p>This editor is based on Prosemirror, fully extendable and renderless. You can easily add custom nodes as Vue components.</p>',
       extensions: [
         new Document(),
         new Paragraph(),
@@ -56,6 +57,7 @@ export default {
         new Bold(),
         new Italic(),
         new Code(),
+        new Heading(),
       ],
     })
   },