Check unknown target properties in new `@Ignored` annotation

Nobody has claimed this yet.

Referencing a target that does not exist results in a compilation error.
Therefore we should support the check of target properties (`MapstructTargetReference`) for that annotation.

```java
interface FooMapper {

@Ignored(targets = "nonExistent") // mark this as unknown
CarDto map(Car car);
}
```

This should also work for nested properties like `@Ignored(targets = "driver.name")`.

Beware that writing it as an array must also work:
```java
@Ignored(targets = { "a", "b" })
```

Start by locating MapstructTargetReference and the handling of targets in the @Ignored annotation. Verify unknown simple targets, nested properties such as driver.name, and array syntax with multiple targets; done means each invalid target is reported as a compilation error.

Written by the indexing model from the issue text.