fix(core): isNodeSelection, isTextSelection not always false (#3089)

Fixes #2979.  Since the ProseMirror TypeScript upgrade, these have
always returned false, since the Selection type tree are all classes
now.

packages/core/src/helpers/isTextSelection.ts

@@ -1,7 +1,5 @@
 import { TextSelection } from 'prosemirror-state'
 
-import { isObject } from '../utilities/isObject'
-
 export function isTextSelection(value: unknown): value is TextSelection {
-  return isObject(value) && value instanceof TextSelection
+  return value instanceof TextSelection
 }