Use `<expr>` nmap for sneak#cancel mapping

Use <expr> map to avoid remapping of e.g. ':', but return <esc>, which needs to get remapped (for e.g. `<Home>`).

I am using the following mappings, and using `Esc` or e.g. `Home` (which uses an escape code) while using sneak would trigger the warning message:

    nnoremap . :
    vnoremap . :
    nnoremap : :echohl WarningMsg \| echomsg "STOP USING SHIFT!" \| echohl None \| sleep 500m<cr>: