plugins/whichkey: Support window and layout fields

[alexpetrean80]

In lua wc.setup can take some optional parameters that can change the way the helper looks, e.g.

wc.setup({
  window = { margin = {1, 0, 0, 0.8 }, border = "single"},
  layout = { height = {min = 4, max = 150}, align = "left" },
})

That seem to be missing from nixvim.plugins.which-key and they would be nice to have.

[alexpetrean80]

After looking over plugins/utils/which-key I realised the options exist, they are just not documented.

[khaneliman]

Closing as supporting plugin's options in #1990 and also allowing undocumented
options.