Warn when there's a binding that applies to multiple bindable types

[lmckenzi]

Based on HL7/fhir-ig-publisher#384, we've created https://jira.hl7.org/browse/FHIR-48480 to add a warning if a user creates a binding that will end up applying to multiple types (e.g. string + Quantity + CodeableConcept) as this is rarely the intension. However, the tracker will only address the issue for R6 on. It would be good if the validator would also flag this issue for earlier models.