With
let g:grepper.prompt_quote = 2
under Windows 10, no results are found. That is, if the search term is surrounded by single quotes, then, under Windows 10, no grepper tool (be it git-grep, rg, ag, ...) finds any result.
Instead double quotes work fine.
Therefore
let g:grepper.prompt_quote = has('win32') ? 3 : 2
is to be recommended.