Rename RenderContext to EditorContext in both libraries. 

Rationale:

In a previous ticket (MGNLFE-18) we renamed the packages from 'react-renderer' and 'angular-renderer' to 'react-editor' and 'angular-editor'. 

However the Contexts are still named 'RendererContext', this terminology is confusing when used in a project because a developer does not know what 'Renderer' is referring to. (Generally React or Angular themselves are thought of as the renderer.)

AC: Implement deprecation

We must be able to evolve and deprecate early APIs and namings gradually, then we can do 1.0.1s or 1.1.0s.
—regardless of whether it's at the cost of duplicate component classes, or if we find deprecation utils, compatibility code generation, etc. I expect more changes will surface in time, so we'll have to go down that road anyway.

With proper deprecation messages, change logs, early adopters will adjust pretty fast.