Ignore Razor design-time files in EnC

[jinujoseph]

Razor produces design-time files to support tooling. We need to make sure these experiences aren't broken so that tooling keeps working they way they are today, while we support EnC

The current thinking is that we would use analyzer config file setting. Essentially, Razor nuget package would contain msbuild targets file that adds a root-level analyzer config file of a well known name (RazorSourceGenerator.razorencconfig) to the compilation, but only for design-time vs builds (there is an msbuild property that can be used for this condition). Razor source generator has access to any such config files. It would use it to decide which form of the source files it should generate (design-time file/runtime file). EnC would also insert this config file into the EnC compilation to force it to generate runtime files.

Removing this workaround is tracked by #51678

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.