patch 8.1.0281: parsing command modifiers is not separated

Problem:    Parsing command modifiers is not separated.
Solution:   Move command modifier parsing to a separate function.
diff --git a/src/feature.h b/src/feature.h
index efe349d..6ae3ac2 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -355,6 +355,10 @@
 # endif
 #endif
 
+#ifdef FEAT_EVAL
+# define HAVE_SANDBOX
+#endif
+
 /*
  * +profile		Profiling for functions and scripts.
  */