Legacy editor shim layer

[DustinCampbell]

This change adds a shim layer for the Razor legacy editor to insulate the legacy editor from upcoming structural and API changes.

The approach taken here is to expose new internal types to the legacy editor via an "external access" assembly. Those internal types wrap existing Razor types without exposing them. Most of the "magical code" is in RazorWrapperFactory.cs. Care has been taken to avoid exposing any of the dependencies of that assembly so that the legacy editor doesn't transitively reference the compiler or tooling layers.

Next steps:

• Once this is merged, we can follow up with a change in Web Tools (already prepared) to use the new shim. Until that change is made, the legacy editor will continue to work against the existing Razor types.
• Add more tests for the shim layer to build confidence when we refactor the compiler.
• Further work could be done to simplify the contract with the legacy editor. In its current state, the shim really just wraps objects that the legacy editor uses. The legacy editor uses those wrapper objects directly to build completion lists, classify tokens, etc. However, with further refactoring of Web Tools we could provide a higher level shim API. It's not clear if that'll be needed.

[333fred]

Overall looking good. A few questions/suggestions.

[DustinCampbell]

@333fred: I believe I've addressed all of your feedback. Let me know if you have any other concerns. Thanks!