ast: Making partial object key rules contribute to dynamic portion of object type (#6177)

... instead of static portion.

This change allows object unification/comparison between an object composed at eval-time (i.e. constructed by rules) and a static object that doesn't contain all keys declared by rules, as those might not be defined at eval-time.

Fixes: #6138
Signed-off-by: Johan Fylling <johan.dev@fylling.se>