Advanced RuleSets

[cmoesel]

Consider introducing support for advanced RuleSet syntax with optional parameters, default values, etc. This has been discussed in the past but did not receive enough community support at the time to justify the investment.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Proposal.3A.20Advanced.20RuleSet.20Syntax/near/342428945

NOTE: This corresponds with fhir-shorthand#184.