BufReadPost does not trigger when editing a nonexistent file, so
language servers would not automatically attach when editing a new file
if autostart was disabled or if the server had no associated filetypes.
This fixes that by adding BufNewFile to autocommands hooked to
BufReadPost.

Sometimes, BufNewFile triggers before 'filetype' is set. Using
vim.schedule() should ensure filetype detection runs before the
callback.

Currently avoids an issue where a `FileType` autocommand schedules a
call to one of these functions but the buffer gets wiped out before the
next event loop. `nvim_get_option_value()` with the `filetype` argument
can cause this as it creates a transient buffer that only lasts for a
single function call.