patch 8.1.1886: command line expansion code is spread out

Problem:    Command line expansion code is spread out.
Solution:   Move the code to cmdexpand.c. (Yegappan Lakshmanan, closes #4831)
diff --git a/src/proto.h b/src/proto.h
index ab0bee3..02b9642 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -66,6 +66,7 @@
 # include "buffer.pro"
 # include "change.pro"
 # include "charset.pro"
+# include "cmdexpand.pro"
 # include "cmdhist.pro"
 # ifdef FEAT_CSCOPE
 #  include "if_cscope.pro"