move plugins to extensions

packages/core/src/extensions/editable.ts

@@ -0,0 +1,15 @@
+import { Plugin, PluginKey } from 'prosemirror-state'
+import { createExtension } from '../Extension'
+
+export const Editable = createExtension({
+  addProseMirrorPlugins() {
+    return [
+      new Plugin({
+        key: new PluginKey('editable'),
+        props: {
+          editable: () => this.editor.options.editable,
+        },
+      }),
+    ]
+  },
+})