Ability to construct Microstates with explicit external references.

[cowboyd]

The rationale for references can be found at https://beta.observablehq.com/@cowboyd/microstate-variables

This issue is for constructing microstates by hand hand with references to external objects.

https://beta.observablehq.com/@cowboyd/microstate-variables#explicit

It does not include passing references from one microstate to another.