feat: Support name_formatter for unnamed buffers

[mskelton]

This adds a new config option unnamed_title to support changing the name of unnamed buffers from [No Name] to anything the user wants (e.g. Untitled).

[akinsho]

@mskelton I think you can accomplish this by using the name_formatter field you can just specify a function and if the path of the buffer passed in is an empty string then use whatever name you want

[mskelton]

name_formatter is not called for unnamed buffers.

  local name = "[No Name]"
  if buf.path and #buf.path > 0 then
    name = fn.fnamemodify(buf.path, ":t")
    name = is_directory and name .. "/" or name
    if buf.name_formatter and type(buf.name_formatter) == "function" then
      name = buf.name_formatter({ name = name, path = buf.path, bufnr = buf.id }) or name
    end
  end

[akinsho]

@mskelton I think the solution then IMO would be adapting the logic to work with no name buffers as well so there's one versatile solution rather than two

[mskelton]

@akinsho I changed to allow name_formatter to work with unnamed buffers.

[akinsho]

LGTM 👍🏾 thanks for making the changes