fix: add expression chains in Observer Maps schema

[janechu]

🐛 Bug Report

When a path in a FAST HTML template is only present in an expression chain such as for an f-when it does not create a _Changed on the constructor or get added to the generated schema.

💻 Repro or Code Sample

<f-when value="{{ a.b }}"></f-when>

🤔 Expected Behavior

As with all paths, this should be added to the generated JSON schema as an object "a" with property "b".

🔦 Context

Currently attempting to use this functionality to hide/show a button.