refactor(filetype)!: allow vim.filetype.match to use different strategies (#18895)

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)`

runtime/filetype.lua

@@ -12,7 +12,7 @@ vim.api.nvim_create_augroup('filetypedetect', { clear = false })
 vim.api.nvim_create_autocmd({ 'BufRead', 'BufNewFile' }, {
   group = 'filetypedetect',
   callback = function(args)
-    local ft, on_detect = vim.filetype.match(args.file, args.buf)
+    local ft, on_detect = vim.filetype.match({ buf = args.buf })
     if ft then
       vim.api.nvim_buf_set_option(args.buf, 'filetype', ft)
       if on_detect then