Script refactoring, frontend tests: changes according to semantic changes in scripts

Add checker tests for scripts

compiler/testData/diagnostics/tests/script/varInScript.txt

@@ -2,7 +2,6 @@ package
 
 public final class Script {
     public constructor Script()
-    public final val rv: kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public final fun main(): kotlin.Boolean