refactor(pages): migrate pages collection config to built-in type-safe custom field

[jhb-dev]

⚠️ This PR requires Payload ^3.65.0 for type-safe custom field support. Will merge after allowing time for users to upgrade.

Summary

Refactors page collection configuration to use Payload's build-in type-safe custom field pattern via module augmentation, replacing the previous approach of using a custom PageCollectionConfig type for extending Payload's CollectionConfig type.

Breaking Changes

Before:

import { PageCollectionConfig } from '@jhb.software/payload-pages-plugin'

export const Pages: PageCollectionConfig = {
  slug: 'pages',
  page: {
    parent: { collection: 'pages', name: 'parent' },
    isRootCollection: true,
  }
 // ...
}

After:

import type { CollectionConfig } from 'payload'

export const Pages: CollectionConfig = {
  slug: 'pages',
  custom: {
    pagesPlugin: {
      page: {
        parent: { collection: 'pages', name: 'parent' },
        isRootCollection: true,
      },
    },
  },
  // ...
}