Made obsolete by now graduated `filetype.lua` (enabled by default).

Note that changes or additions to the filetype detection still need to
be made through a PR to vim/vim as we port the _logic_ as well as tests.

Problem: filetype detection does not run on piped input
Solution: add `StdinReadPost` to main filetype.lua autocommand

Rationale: legacy filetype detection checked contents by sourcing
`scripts.vim` in separate autocommands, including on `StdinReadPost`.
For Lua filetype detection, this was moved into the main autocommand,
with bundled `scripts.vim` gated behind `g:do_legacy_filetype` (i.e.,
only user `scripts.vim` are sourced for compatibility by default). Adding
`StdinReadPost` to the main autocommand again runs content checks on
piped input without requiring code duplication and low-payoff
refactoring.

* 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).