Normalize selectors to not be a mixed type

Currently our `target` selectors are a mixed type. It can be one of:

1. string
2. array of strings
3. both

This works fine in loosely typed languages, but in strictly type languages like Java and C# that doesn't work. We should change the selectors so it is a consistent type.

Start by tracing how axe-core represents and consumes `target` selectors; the issue names no files, tests, or entry points. Determine the consistent selector type required by the Java and C# integrations, then verify that string, string-array, and mixed inputs have one documented representation and matching behavior.

Written by the indexing model from the issue text.