Provide more utility functions (pick, ...)

There's a number of common utility functions which are useful in Contentlayer projects (e.g. a type-safe `pick` function) like below or [here](https://github.com/leerob/leerob.io/blob/6f4ddbb390ca513dab32ecc75ce6fc8b05b9902b/lib/utils.ts#L1):

We should provide these utility functions out of the box with Contentlayer. Candidates:

- `pick`: Type-safe function similar to TS `Pick` type
- A function that helps with getting the "next" and "previous" document (common in docs pages)
- A function that helps with sorting documents
- `findById`

If you'd like to see an other utility function added, please share your requests/ideas in the comments below. 🙏

未指定文件、测试或入口点。首先检查现有的 Contentlayer utility API 以及这里列出的候选函数，然后确定应接受哪些 utility 和行为。完成的标准是，已实现约定的 utility 集合，并覆盖其文档中说明的行为。

由索引模型根据 Issue 内容生成。