Add `Union` Field Type

Basically, allow me to define a union of types that a field can be.

Simple example would be `string | string[]`. Config proposal:

```typescript
// ...
author: {
type: `union`,
of: [
{
type: `string`
},
{
type: `list`,
of: {
type: `string`
}
}
]
},
```

This would permit the following frontmatter:

```yaml
author: Foo Bar
# or
author:
- Foo Bar
- Baz Qux
```

If this is already possible with the existing field types, please add examples to the documentation which demonstrate how to do this, as it's not readily apparent how this can be done.

首先查看现有的字段类型和文档示例，因为 issue 没有明确 unions 是否已经可用。定义 union 字段应如何针对标量值和列表值进行配置和验证，或者使用给出的 frontmatter 示例记录现有方法；当其中一种路径得到明确支持时，即可视为完成。

由索引模型根据 Issue 内容生成。