The package is commonjs, its type definition for ESM package is wrong

Related #2209

cc @jasoniangreen

**What version of Ajv are you using? Does the issue happen if you use the latest version?**

**Your typescript code**

```typescript

```

**Typescript compiler error messages**

```

```

**Describe the change that should be made to address the issue?**

**Are you going to resolve the issue?**