[picker] args option for overriding

[smilovanovic]

At the moment, args option for file picker is extending the default command args.
It would be useful to have another option to override default args.

snacks.nvim/lua/snacks/picker/source/files.lua

Line 110 in 6e0962a

vim.list_extend(args, opts.args or {})

One usage example would be to search only directories using file picker and fd command

{
  cmd = "fd",
  args = {"-t", "d"}
}

With that we can chain two file pickers, first one to select the search directory and the second one to live grep in selected dir.
Thoughts?

[folke]

#1023 (comment)