feature request: set `filetype` for REPL buffer

[wangl-cc]

Set filetype for REPL buffer will be helpful for being detected by iron and other plugins. For example, toggle REPL with IronRepl will cause an error at REPL buffer. Thus, I set it filetype to iron and check it like this:

if vim.bo.filetype == "iron" then
  vim.api.nvim_win_hide(0)
else
  iron.repl_for(vim.bo.filetype)
end

Currently, I set filetype in repl_open_cmd. But I think it's an abusing, because it's designed to setup windows instead of buffer. It may be better to be specified when the REPL buffer is created.