move dropcursor and gapcursor to its own package

packages/core/src/plugins/index.ts

@@ -1,14 +1,10 @@
 import { keymap } from 'prosemirror-keymap'
 import { baseKeymap } from 'prosemirror-commands'
-import { dropCursor } from 'prosemirror-dropcursor'
-import { gapCursor } from 'prosemirror-gapcursor'
 import { undoInputRule } from 'prosemirror-inputrules'
 import editable from './editable'
 import focus from './focus'
 
 export default [
-  () => dropCursor(),
-  () => gapCursor(),
   () => keymap({ Backspace: undoInputRule }),
   () => keymap(baseKeymap),
   editable,