Define relativize as a built-in UI helper
To make it easier on UI development, the relativize helper should be a built-in helper (i.e., provided out of the box). If the UI defines a relativize helper, the helper from the UI will still win. However, since relativize is really part of the model, and the core operation of Antora, it should at least be offered out of the box. It also keeps the logic DRY.