patch 8.1.1823: command line history code is spread out

Problem:    Command line history code is spread out.
Solution:   Put the code in a new file. (Yegappan Lakshmanan, closes #4779)
            Also graduate the +cmdline_hist feature.
diff --git a/src/proto.h b/src/proto.h
index 34ef9cc..f6f5319 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -65,6 +65,7 @@
 # include "buffer.pro"
 # include "change.pro"
 # include "charset.pro"
+# include "cmdhist.pro"
 # ifdef FEAT_CSCOPE
 #  include "if_cscope.pro"
 # endif