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)

src/misc2.c

@@ -2530,6 +2530,7 @@ static struct key_name_entry
     {K_PLUG,		(char_u *)"Plug"},
     {K_CURSORHOLD,	(char_u *)"CursorHold"},
     {K_IGNORE,		(char_u *)"Ignore"},
+    {K_COMMAND,		(char_u *)"Cmd"},
     {0,			NULL}
     // NOTE: When adding a long name update MAX_KEY_NAME_LEN.
 };