Consider supporting special variables in FSH

A feature suggestion came through Zulip for FSH support for special variables like `CURRENT_DATE`. These would likely be predefined variables that FSH supports, rather than a generic variable substitution.

See Zulip thread: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Dynamic.20value.20substitution.20in.20FSH

Start with the linked Zulip thread to understand the requested special variables and expected substitution behavior. Define which predefined variables, such as CURRENT_DATE, FSH should support and how their values are evaluated; the work is done when that behavior is agreed and implemented with appropriate coverage.

Written by the indexing model from the issue text.