fix: assign filetype to background buffer (#91)

## Summary

Assigns a filetype (`zenmode-bg`) to the background buffer created by
zen-mode.

> Why?

I use a
[utility](https://github.com/ditsuke/nvim-config/blob/main/lua/ditsuke/utils/ui.lua#L21-L45)
to close floating windows. This utility interferes with zen-mode's
background window so it helps if it has a filetype I can use to filter
it out.

Author: ditsuke

lua/zen-mode/view.lua

@@ -156,6 +156,7 @@ function M.create(opts)
   M.plugins_on_open()
 
   M.bg_buf = vim.api.nvim_create_buf(false, true)
+  vim.api.nvim_buf_set_option(M.bg_buf, "filetype", "zenmode-bg")
   local ok
   ok, M.bg_win = pcall(vim.api.nvim_open_win, M.bg_buf, false, {
     relative = "editor",