Support Descendents

[cmoesel]

The Descendents operator was introduced to support [FHIRPath descendants(https://hl7.org/fhirpath/#descendants-collection) but is accessible from standard CQL via the following syntax:

SomeTupleOrList.descendents()

It seems it is seldom used, but we should consider supporting it.

NOTE: The CQL spec spells it descendents, but the FHIRPath spec spells it descendants. The CQL translator only supports the spelling: descendents.