fix: add distinct validate to richtext field type definition

[akhrarovsaid]

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:

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