Incorrect typing for `enum` on computed fields

currently when defining a computed field of type `enum` like this:

```js
import { defineDocumentType, makeSource } from 'contentlayer/source-files'

const locales = ['en', 'de'] as const

const Post = defineDocumentType(() => {
return {
name: 'Post',
filePathPattern: 'posts/**/*.mdx',
fields: {
language: {
type: 'enum',
options: locales,
required: true
}
},
computedFields: {
locale: {
type: 'enum',
options: locales,
resolve(doc) {
return doc['language']
}
}
}
}
})

export default makeSource({
contentDirPath: 'content',
documentTypes: [Post]
})
```

this generates a ts schema:

```ts
export type Post = {
/** File path relative to `contentDirPath` */
_id: string
_raw: Local.RawDocumentData
type: 'Post'
language: 'en' | 'de'
/** Markdown file body */
body: Markdown
locale: enum
}
```

note that the regular `enum` field is correct, but the computed `enum` field is of type `enum`

i guess this is because of https://github.com/contentlayerdev/contentlayer/blob/dcc48858f7643f882a72a9cc5941deb36bbb1ecf/packages/%40contentlayer/core/src/generation/generate-types.ts#L120

---

steps to reproduce:
- `git clone git@github.com:stefanprobst/issue-contentlayer-computed-field-types.git`
- `cd issue-contentlayer-computed-field-types`
- `npm i`
- `npm run generate`
- `cat .contentlayer/generated/types.d.ts`

Start with packages/@contentlayer/core/src/generation/generate-types.ts, especially the area linked in the issue, and reproduce the problem with the listed npm install, npm run generate, and cat commands. Verify that the generated .contentlayer/generated/types.d.ts gives the computed locale field the 'en' | 'de' type instead of the literal enum type.

由索引模型根据 Issue 内容生成。