add tests to isClass

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

@@ -0,0 +1,19 @@
+/// <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)
+  })
+})