SetOptionalDeep

### Type description + examples

Like `SetOptional` but works on deeply-nested properties

```typescript
type Foo = { bar: { baz: string } }
type Example = SetOptionalDeep<Foo, "bar.baz">
// Example = { bar: { baz?: string } }
```

### Type source

_No response_

### Search existing types and issues first

- [x] I tried my best to look for it