Fix to syntax highlighting breaking (in lua)

I dunno if this has been said before, but I'll just put it here:
When GoyoLeave, check for filetype and source the syntax file (this can be converted to vimscript)
``` lua
function au(evt, pat, cmd) -- (string|table), (string|table), (string)
vim.api.nvim_create_autocmd(evt, { pattern = pat, command = cmd, })
end

au("User", "GoyoLeave", [[if &filetype == "markdown" | source /Users/deangao/.config/nvim/syntax/markdown.vim | endif]])
au("User", "GoyoLeave", [[if &filetype == "nroff" | source /Users/deangao/.config/nvim/syntax/nroff.vim | endif]])
```

Start at the GoyoLeave event handling and reproduce the syntax-highlighting break described for Markdown and nroff buffers. Compare the issue's autocmd workaround with the plugin's behavior after leaving Goyo; done means syntax highlighting is restored for those filetypes without the user-local source commands.

Written by the indexing model from the issue text.