Code quality: Fix header guards consistency

Adds `header_guards.sh` bash script, used in CI to validate future
changes. Can be run locally to fix invalid header guards.

platform/javascript/export/export.h

@@ -33,4 +33,4 @@
 
 void register_javascript_exporter();
 
-#endif
+#endif // JAVASCRIPT_EXPORT_H

platform/javascript/export/export_plugin.h

@@ -144,4 +144,4 @@ public:
 	~EditorExportPlatformJavaScript();
 };
 
-#endif
+#endif // JAVASCRIPT_EXPORT_PLUGIN_H

platform/javascript/export/export_server.h

@@ -248,4 +248,4 @@ public:
 	}
 };
 
-#endif
+#endif // JAVASCRIPT_EXPORT_SERVER_H