Add a Option, to mark the "default" Binding Property or Attribute (low prio)

I use the CustomElement Manifest in a designer app. In this designer app you can Drag&Drop Properties from your DataModel to an Element. It will then create automaticly a default Binding. For example if you drag on a "input" it will automaticly bind to "value" property.
it would be nice if we could create a "hint" wich you could add to the spec to allow define a default.

Begin with the Custom Element Manifest specification and the existing representation of properties and attributes. Determine how a default-binding hint should be expressed for the input/value example, then confirm the proposal covers both properties and attributes and is documented in the specification.

Written by the indexing model from the issue text.