patch 8.1.1210: support for user commands is spread out

Problem:    Support for user commands is spread out. No good reason to make
            user commands optional.
Solution:   Move user command support to usercmd.c.  Always enable the
            user_commands feature.
diff --git a/src/proto.h b/src/proto.h
index 448233a..e34af85 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -227,6 +227,7 @@
 # endif
 # include "ui.pro"
 # include "undo.pro"
+# include "usercmd.pro"
 # include "userfunc.pro"
 # include "version.pro"
 # include "window.pro"