LibSQL: Add a new Result class to replace SQLResult

The existing SQLResult class predates our TRY semantics. As a result, in
the AST execution methods, there is a lot of is_error checking on values
that could instead be wrapped with TRY. This new class will allow such
semantics, and is also stack allocated (no need to be a RefPtr). It is
heavily based on LibJS's completion class.

Userland/Libraries/LibSQL/Forward.h

@@ -22,6 +22,7 @@ class IndexDef;
 class Key;
 class KeyPartDef;
 class Relation;
+class Result;
 class Row;
 class SchemaDef;
 class SQLResult;