Editor plugin support for ReSharper?

Currently, the editor plugin has many assumptions about being used in Rider, and isn't applicable to being used for the ReSharper plugin. But this means that a number of features (e.g. installing a compatible version of nunit for local unit testing - #335) are unavailable to ReSharper. On the flip side, it also performs a number of tasks that are inappropriate for ReSharper (e.g. setting Rider as the default external editor, simplifying debugging setup, etc.)

This task is initially to investigate if it's worth migrating the editor plugin to also support ReSharper.

* Evaluate what the Rider plugin currently does to see what features are appropriate for ReSharper
* Should we expose the protocol to ReSharper? List the protocol features, see what would be useful, what wouldn't, and what's already handled by VSTU
* If refactoring/migrating the plugin is not an appropriate use of resources, would a stripped down, simpler version be useful?