move plugins to extensions

packages/core/src/extensions/keymap.ts

@@ -0,0 +1,19 @@
+import { keymap } from 'prosemirror-keymap'
+import { baseKeymap } from 'prosemirror-commands'
+import { undoInputRule } from 'prosemirror-inputrules'
+import { createExtension } from '../Extension'
+
+export const Keymap = createExtension({
+  addProseMirrorPlugins() {
+    return [
+      keymap({ Backspace: undoInputRule }),
+      keymap(baseKeymap),
+    ]
+  },
+})
+
+declare module '../Editor' {
+  interface AllExtensions {
+    Keymap: typeof Keymap,
+  }
+}