Switch expressions not supported

Switch statement works fine but not switch expressions. Compilation errors occur when using one. Can be mitigated by using switch statements insead.

Start by reproducing the compilation error with a C# switch expression in FastScriptReload, then trace how the project handles C# compilation and compare it with switch statements. Done means switch expressions compile successfully without requiring a statement-based workaround.

索引モデルが issue の本文から書いたものです。