feat(picker): add custom action to setqflist

[S1M0N38]

The default action for vim.fn.setqflist is to create a new list.
From the help:

If {action} is not present or is set to ' ', then a new list
is created. The new quickfix list is added after the current
quickfix list in the stack and all the following lists are
freed.

It would be nice if

https://github.com/folke/snacks.nvim/blob/bc0630e43be5699bb94dadc302c0d21615421d93/lua/snacks/picker/actions.lua#L420C1-L444C4

let you specify a custom action while keeping the " " as default.

For example

---@param items snacks.picker.Item[]
---@param opts? {win?:number}
---@param action? string
local function setqflist(items, opts, action)
  local qf = {} ---@type vim.quickfix.entry[]
  for _, item in ipairs(items) do
    qf[#qf + 1] = {
      filename = Snacks.picker.util.path(item),
      bufnr = item.buf,
      lnum = item.pos and item.pos[1] or 1,
      col = item.pos and item.pos[2] + 1 or 1,
      end_lnum = item.end_pos and item.end_pos[1] or nil,
      end_col = item.end_pos and item.end_pos[2] + 1 or nil,
      text = item.line or item.comment or item.label or item.name or item.detail or item.text,
      pattern = item.search,
      valid = true,
    }
  end
  if opts and opts.win then
    vim.fn.setloclist(opts.win, qf, action or " ")
    vim.cmd("botright lopen")
  else
    vim.fn.setqflist(qf, action or " ")
    vim.cmd("botright copen")
  end
end

---

With the current <C-Q>, the user cannot add to the qflist items from different sources (e.g. diagnostic and buffer) to the same qflist. The same can be said for loclist.

[S1M0N38]

This issue was open before #1539.