Make userVersion and useInsertReturning constants

Not quite sure whether it may make sense to set
userVersion?
If that becomes an issue, we can still generate
`nonisolated(unsafe)`.

Plugins/Libraries/LighterGeneration/RecordGeneration/GenerateDatabaseStruct.swift

@@ -68,7 +68,7 @@ extension EnlighterASTGenerator {
 
     // Schema version
 
-    typeVariables.append(.var(
+    typeVariables.append(.let(
       public: options.public, "userVersion",
       is: .integer(database.userVersion),
       comment: "User version of the database (`PRAGMA user_version`)."
@@ -84,7 +84,7 @@ extension EnlighterASTGenerator {
 
     // Whether SQLite3 supports returning (the user can override!)
 
-    typeVariables.append(.var(
+    typeVariables.append(.let(
       public: options.public, "useInsertReturning",
       is: .cmp(
         .call(name: "sqlite3_libversion_number"),