…tics reported by the C# compiler. At some point, we should consider doing the same for Razor diagnostics, but that can be done as a future step.

…ffect parsing recovery.

We were not consistently handling recovery after seeing `@` signs in code blocks, which meant that there were all sorts of interesting scenarios where an escaped C# identifier would cause odd error recovery. There are a few edge cases that are now compile errors, but this is a more consistent overall experience. Fixes dotnet#10186 as well.