patch 8.2.1262: src/ex_cmds.c file is too big

Problem:    src/ex_cmds.c file is too big.
Solution:   Move help related code to src/help.c. (Yegappan Lakshmanan,
            closes #6506)
diff --git a/src/proto.h b/src/proto.h
index 8c768fc..e3a25f9 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -95,6 +95,7 @@
 # include "gui_xim.pro"
 # include "hardcopy.pro"
 # include "hashtab.pro"
+# include "help.pro"
 # include "highlight.pro"
 # include "indent.pro"
 # include "insexpand.pro"