refactor: rename the atom directory to shell

shell/app/atom_library_main.h

@@ -0,0 +1,23 @@
+// Copyright (c) 2013 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef ATOM_APP_ATOM_LIBRARY_MAIN_H_
+#define ATOM_APP_ATOM_LIBRARY_MAIN_H_
+
+#include "build/build_config.h"
+#include "electron/buildflags/buildflags.h"
+
+#if defined(OS_MACOSX)
+extern "C" {
+__attribute__((visibility("default"))) int AtomMain(int argc, char* argv[]);
+
+#if BUILDFLAG(ENABLE_RUN_AS_NODE)
+__attribute__((visibility("default"))) int AtomInitializeICUandStartNode(
+    int argc,
+    char* argv[]);
+#endif
+}
+#endif  // OS_MACOSX
+
+#endif  // ATOM_APP_ATOM_LIBRARY_MAIN_H_