[5.x] Adds Boolable interface to resolve issues with ArrayableString instances in conditions under some circumstances.

[JohnathonKoster]

This PR fixes #10589 and replaces #10593 and implements the idea in this comment #10593 (comment). The root cause of the linked issue is that passing values to a tag using the :some_var="var_name" syntax removes the wrapper Value object. In the case of LabeledValue, this will ultimately resolve a value similar to the following when doing the "truthiness" check:

array:3 [
  'value' => null,
  'label' => null,
  'key' => null
]

This PR introduces a new Boolable interface which is now used by the underlying ArrayableString to help Antlers make decisions when performing truthiness checks.