refactor: Use named exports instead of default exports (#2238)

* use named exports instead of default exports

* fix tests

Co-authored-by: Philipp Kühn <philippkuehn@MacBook-Pro-von-Philipp.local>

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

@@ -1,7 +1,7 @@
 /// <reference types="cypress" />
 
 import { Extension } from '@tiptap/core/src/Extension'
-import getExtensionField from '@tiptap/core/src/helpers/getExtensionField'
+import { getExtensionField } from '@tiptap/core/src/helpers/getExtensionField'
 
 describe('extend extensions', () => {
   it('should define a config', () => {