Add support for _Imports.razor

[rynowak]

Design Notes: https://github.com/aspnet/specs/blob/master/notes/ViewImports_in_the_components_world.md

Partially blocked by: #4057

• Add support _Imports.razor and tests
• Make _ViewImports.cshtml apply to views and _Imports.razor apply to components
• Treat _Imports.razor as a distinct FileKind in SDK/Tooling
• Limit _ViewImports.cshtml to directives that apply to views, Limit _Imports.razor to directives that apply to components
• Make it an error to include code, markup, and block directives in _Imports.razor
• Make sure generated class from _Imports.razor doesn't show up in compiled code

[rynowak]

I've broken down the feature with a rough breakdown that makes sense to me. Who implements this feature can divide it however they want, this is just a suggestion 😆