Expose link validation API in DocumentCursor

[jenshalm]

Currently the validation logic is mostly hidden inside Laika's built-in rewrite rules. However, users might also create new nodes that contain links in a directive or rewrite rule, and while DocumentCursor has all the data required to validate the link, repeating the logic for every use case would be a lot of boilerplate. It's also slow as link validation requires node traversal through a document to find a matching id for the link target, which is inefficient if results are not cached.