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/terminal.c b/src/terminal.c
index e81754c..0fb9f48 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -2180,6 +2180,10 @@
 		    return FAIL;
 		}
 	    }
+	    break;
+
+	case K_COMMAND:
+	    return do_cmdline(NULL, getcmdkeycmd, NULL, 0);
     }
     if (typed)
 	mouse_was_outside = FALSE;