LibSQL: Move Order and Nulls enums from SQL::AST to SQL namespace

The Order enum is used in the Meta component of LibSQL. Using this enum
meant having to include the monster AST/AST.h include file. Furthermore,
they are sort of basic and therefore can live in the general SQL
namespace. Moved to LibSQL/Type.h.

Also introduced a new class, SQLResult, which is needed in future
patches.

Userland/Libraries/LibSQL/Forward.h

@@ -22,6 +22,7 @@ class IndexDef;
 class Key;
 class KeyPartDef;
 class Row;
+class SQLResult;
 class TableDef;
 class TreeNode;
 class Tuple;