<trigger> (currently unsupported) incorrectly treated as ModelValueNode

[eyelidlessness]

Filing a quick issue so we have something to track. This is definitely a bug!

In chat we're discussing filling in gaps in unsupported question types, by parsing them to stub nodes with unknown for anything pending further design. I know there are a few other cases, and expect they'll have the same behavior.

Will fill in more details if/as they come up, otherwise the intent of this issue is to track:

1. Do not produce ModelValueNode for form nodes associated with known body elements
2. Do produce something for those nodes, which for now will just be a node type which clients can't do anything with (other than observe their presence)