JSSpecCompiler: Add reference resolving pass

It replaces UnresolvedReference with Variable, FunctionPointer, or
SlotName nodes. Also, it gathers all variable names from their
declarations.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Function.cpp

@@ -13,6 +13,7 @@ Function::Function(ExecutionContext* context, StringView name, Tree ast)
     : m_context(context)
     , m_name(name)
     , m_ast(move(ast))
+    , m_return_value(make_ref_counted<VariableDeclaration>("$return"sv))
 {
 }