patch 8.2.1978: making a mapping work in all modes is complicated

Problem:    Making a mapping work in all modes is complicated.
Solution:   Add the <Cmd> special key. (Yegappan Lakshmanan, closes #7282,
            closes 4784, based on patch by Bjorn Linse)
diff --git a/src/edit.c b/src/edit.c
index 934f6c1..d92a951 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -1031,6 +1031,10 @@
 	case K_IGNORE:	// Something mapped to nothing
 	    break;
 
+	case K_COMMAND:		// <Cmd>command<CR>
+	    do_cmdline(NULL, getcmdkeycmd, NULL, 0);
+	    break;
+
 	case K_CURSORHOLD:	// Didn't type something for a while.
 	    ins_apply_autocmds(EVENT_CURSORHOLDI);
 	    did_cursorhold = TRUE;