* revert to filetype.vim by setting `g:do_legacy_filetype`
* skip either filetype.lua or filetype.vim via `g:did_load_filetypes`

(Running both is no longer required and therefore no longer supported.)

For example on MacOS, /etc/hostname.file is symlinked to
/private/etc/hostname.file. We only care about the original file path though.

This enables vim.filetype.match to match based on a buffer (most
accurate) or simply a filename or file contents, which are less accurate
but may still be useful for some scenarios.

When matching based on a buffer, the buffer's name and contents are both
used to do full filetype matching. When using a filename, if the file
exists the file is loaded into a buffer and full filetype detection is
performed. If the file does not exist then filetype matching is only
performed against the filename itself. Content-based matching does the
equivalent of scripts.vim, and matches solely based on file contents
without any information from the name of the file itself (e.g. for
shebangs).

BREAKING CHANGE: use `vim.filetype.match({buf = bufnr})` instead 
of `vim.filetype.match(name, bufnr)`

Many filetypes from filetype.vim set buffer-local variables, meaning
vim.filetype.match cannot be used without side effects. Instead of
setting these buffer-local variables in the filetype detection functions
themselves, have vim.filetype.match return an optional function value
that, when called, sets these variables. This allows vim.filetype.match
to work without side effects.

In `filetype.lua`, source runtime `ftdetect` scripts within the `filetypedetect` augroup, same as `filetype.vim` (and only do so if `g:did_load_ftdetect` does not exist).

When filetype.vim is disabled, create the autocommand that runs
scripts.vim in filetype.lua.

Filetype detection runs on BufRead and BufNewFile autocommands, both of
which can fire without an underlying buffer, so it's incorrect to use
<abuf> to determine the file path. Instead, match on <afile> and assume
that the buffer we're operating on is the current buffer. This is the
same assumption that filetype.vim makes, so it should be safe.