Add "editor" and "standalone" feature tags

These feature tags can be used to check whether the project was
started from an editor binary or from an export template binary.

core/os/os.cpp

@@ -577,6 +577,13 @@ bool OS::has_feature(const String &p_feature) {
 	if (p_feature == "release")
 		return true;
 #endif
+#ifdef TOOLS_ENABLED
+	if (p_feature == "editor")
+		return true;
+#else
+	if (p_feature == "standalone")
+		return true;
+#endif
 
 	if (sizeof(void *) == 8 && p_feature == "64") {
 		return true;