Add object literal syntax to expression language

[jlowin]

The expression grammar currently supports numbers, strings, booleans, null, and identifiers as primary values — but not object literals. You can't construct a {"from": ..., "to": ...} or similar structure inside a {{ }} template expression.

This would be useful for building structured values reactively, e.g. constructing a date range from separate state keys:

{{ {"from": startDate, "to": endDate} }}

The expression engine already preserves types for sole {{ expr }} templates, so an object literal would flow through as a proper object to the renderer.