LibJS: Make Executable ref-counted and let instruction iterator co-own it

This ensures that the instruction stream pointed at by the instruction
iterator remains valid as long as the iterator exists.

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -30,7 +30,7 @@ public:
         Function,
         Block,
     };
-    static CodeGenerationErrorOr<NonnullOwnPtr<Executable>> generate(ASTNode const&, FunctionKind = FunctionKind::Normal);
+    static CodeGenerationErrorOr<NonnullRefPtr<Executable>> generate(ASTNode const&, FunctionKind = FunctionKind::Normal);
 
     Register allocate_register();