fix: add distinct validate to richtext field type definition (#15069)

### What?
This PR adds the `RichTextFieldValidation` type to the `validate`
property of the `RichTextField`.

### Why?
Previously, adding the default `richText` validate function to a typical
`type: 'richText'` field would result in a TS error due to mismatching
types, specifically on the `options` argument. With these changes, you
can now simply thread the default `richText` validation through to the
`validate` property as expected with no unnecessary type manipulation.

Previously, this would result in a TS error:
```ts
import type { CollectionConfig } from 'payload'

import { richText } from 'payload/shared'

export const PostsCollection: CollectionConfig = {
  slug: 'posts',
  // ...
  fields: [
    // ...
    {
      name: 'content',
      type: 'richText',
      validate: richText, // Type error here
    },
  ],
}
```
With the changes introduced here, the TS error is resolved.

### How?
By threading the existing `RichTextFieldValidation` type to the
`validate` property of the `RichTextField`.

Fixes #15068