feat: add support for if, for, assign and merge

[ruscoder]

Refs #1 #2 #3

[ruscoder]

@ir4y it's fully backward compatible, I removed hardcoded R4 Model from resolveTemplate function, now it's passed in service instead.

[ruscoder]

I've added a real-life complex test with a real mapper that accepts: Patient, QuestionnaireResponse, Organization and list of Provenances with associated target resources.

1. It creates an observation and N conditions checking if already existing resources were created before based on Provenances
2. Then it creates Provenances for them to track changes
3. Deletes provenance and resources that weren't mentioned in this mapper

It's one of the most complex mappers I've ever written, so, it proves that FHIRPathMappingLanguage is good enough for complex things.