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.

tests/cypress/integration/core/isClass.spec.ts

@@ -1,19 +0,0 @@
-/// <reference types="cypress" />
-
-import { isClass } from '@tiptap/core/src/utilities/isClass'
-
-describe('isClass', () => {
-  it('returns true for classes', () => {
-    const instance = new (class {
-      public foo = 'bar'
-    })()
-
-    expect(isClass(instance)).to.eq(true)
-  })
-
-  it('return false for objects', () => {
-    const instance = {}
-
-    expect(isClass(instance)).to.eq(false)
-  })
-})