feat($compile): add one-way binding to the isolate scope definition

This change allows the developer to bind an isolate scope / controller property
to an expression, using a `<` binding, in such a way that if the value of the
expression changes, the scope/controller property is updated but not the
converse.

The binding is implemented as a single simple watch, which can also provide
performance benefits over two way bindings.

Closes #13928
Closes #13854
Closes #12835
Closes #13900