Add DesignTime code generation support for TagHelpers.

- We now generate a subset of the TagHelper runtime code during DesignTime. This enables users to see errors in the editor
- Added tests to validate design time code generation.
- Refactored runtime code generation tests (we now use a lot of their infrastructure for the design time pieces).

#208
We copied nearly 90% of the runtime test infrastructure so I just refactored the 2 runtime tests that we pulled it from to fit the format of the design time code generation tests.