Make “and” and “or” helpers support variable argument lengths

In the course of a discussion about this on the Antora users channel, it was discovered that the default UI’s implementations of and and or helpers behave differently than the handlebars helpers in accepting only two arguments. @mojavelinux cited this code: https://github.com/helpers/handlebars-helpers#or

It would be easy enough to reduce over the arguments, although care must be taken to ignore the final argument passed to the helper function, which is some sort of metadata about the helper itself.