Loosen typings for context in Injector interface

[agubler]

Enhancement

The typings for Context of an injector expect it to extends @dojo/core/Evented which although is the standard for Dojo 2 specific injector contexts this restricts interop with external libraries such as Redux as they will not extend Evented.

We should create an extra class/interface as a base for all Injectors that doesn't restrict the context to classes that extends Evented.