Confirm/update Razor compilation on build language

[guardrex]

Razor compilation is now part of every build. This means that Razor compilation issues are caught at design time instead of when the app is first run. Compiling the Razor views and pages also significantly speeds up startup time. And even though your view and pages are built up front, you can still modify your Razor files at runtime and see them updated without having to restart the app.

[guardrex]

@scottaddie This isn't the same as aspnet/Razor#1809, which deals with Razor UI in class libs, but it feels like they perhaps go together on your PR. If u want them separate and don't want this update, then assign it to me, and I'll take care of it.

[guardrex]

Wasn't directly rolled into the issue on #1809, so I'll add it there per #5404 (just saw it noted in the issue).