Fix default main actor isolation. (#200)

* Fix default main actor isolation.

* wip

Author: mbrandonw

Sources/StructuredQueriesMacros/TableMacro.swift

@@ -1474,7 +1474,7 @@ extension TableMacro: MemberMacro {
       }
       """,
       """
-      public struct Selection: \(moduleName).TableExpression {
+      public nonisolated struct Selection: \(moduleName).TableExpression {
       public typealias QueryValue = \(type.trimmed)
       public let allColumns: [any \(moduleName).QueryExpression]
       \(selectionInitializers, separator: "\n")