Feat: add support for checking if there's any complete item selected

[JS-Zheng]

Is your feature request related to a problem? Please describe.
I always use the following snippet to check if there's any complete item be selected (e.g., #262),
However, this feature has been removed in commit 7a50d4.

coc#rpc#request('hasSelected', [])

Many of my coc scripts need this feature, for example,
I like to confirm the completion only when I explicitly select a complete item and then press <CR>:

inoremap <silent><expr> <CR> pumvisible() && coc#rpc#request('hasSelected', [])
                              \? coc#_select_confirm()
                              \: "\<C-g>u\<CR>\<c-r>=coc#on_enter()\<CR>"

Describe the solution you'd like
Provide a coc#is_selected() or coc#_selected() API
or just restore hasSelected action.

[chemzqm]

You can use coc#pum#visible() && coc#pum#info()['index'] != -1