Coc.nvim: Adapt Code Spell Check extension to coc-vim

Is your feature request related to a problem? Please describe.
The spell-check built into vim is incredibly basic: it's meant for prose, not for coding.

I make a ton of typos while programming and one of my favourite extensions for vscode is Code Spell Checker. It works really well and has a fairly large dictionary out of the box.

Describe the solution you'd like
I would love to have that extension adapted to work with coc-vim.

Describe alternatives you've considered
I tried adapting the extension myself but there isn't good documentation of how to do that yet, so I wonder if some maintainer has the goodwill to help with that.