Consider supporting flags on types within Only Rules

[jafeltra]

Consider having the FSH Language support setting flags, like MS, directly on the types that are supplied in an Only Rule. A possibly syntax could look like the following:

* effective[x] only dateTime MS or Period

This was originally suggested on Zulip here: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/How.20do.20I.20declare.20elementdefinition-type-must-support.3F