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

/cc @dougbu 
After rebasing added the `BasicTagHelpers.RemoveTagHelper` to this runtime dataset which makes the two sets differentiate since we don't care about its generation in design time (handled in test below with the `addtaghelper` tests).