Returns |TRUE| when the wildmenu is active and |FALSE|
otherwise. See 'wildmenu' and 'wildmode'.
This can be used in mappings to handle the 'wildcharm' option
gracefully. (Makes only sense with |mapmode-c| mappings).
For example to make work like in wildmode, use:
:cnoremap wildmenumode() ? "<Down><Tab>" : "<c-j>"
(Note, this needs the 'wildcharm' option set appropriately).