fix: add clin sig prop as valid statement proposition

[jsstevenson]

• Add ClinicalSignificance proposition to Statement proposition property annotation
• Refactor out to shared type alias

[korikuzma]

Could we add a comment saying that if SubjectVariantProposition.root gets updated to update Statement.proposition? So that in the future, will remember to update in both places?

[jsstevenson]

Could we add a comment saying that if SubjectVariantProposition.root gets updated to update Statement.proposition? So that in the future, will remember to update in both places?

I refactored out to a separate type alias so that it only gets changed in one place -- tests pass locally, I think this should work?

[korikuzma]

I like it!