Enum values are not validated

I've seen some examples on issues of people using a field of type `enum` to specify the page locale.

```
---
locale: 'en'
---
(...)
```

```ts
defineDocumentType(() => ({
[...],
fields: {
locale: {
type: 'enum',
options: ['en', 'es', 'de'],
required: true
}
}
}));
```

I've just seen on `contentlayer@0.3.0` that if you have a file with an invalid enum value (not in the `options` array), then the build will complete successfully.

```
---
locale: 'not in the options array'
---
(...)
```

```sh
$ yarn contentlayer build
yarn run v1.22.19
$ /(...)/node_modules/.bin/contentlayer build
Generated 2 documents in .contentlayer
Done in 1.32s.
```

I understand that even if it does not validate values, the enum type field is really useful once it automatically generates TS types, but given that we can validate fields with `required`, shouldn't we also be able to validate the if the field value matches what we defined on the schema? (In the case of enum fields)

Beginne damit, das Problem mit einem enum-Feld zu reproduzieren, dessen frontmatter-Wert nicht im options-Array enthalten ist, und verfolge anschließend die Einstiegspunkte für den Contentlayer-Build und die Schema-Validierung. Als erledigt gilt die Aufgabe, wenn der Build den ungültigen Wert ablehnt, während gültige enum-Werte weiterhin Dokumente und TypeScript-Typen generieren.

Vom Indexierungsmodell aus dem Issue-Text verfasst.