feat: filetype.lua (#16600)

Adds a new vim.filetype module that provides support for filetype detection in
Lua.

runtime/filetype.lua

@@ -0,0 +1,22 @@
+if vim.g.did_load_filetypes and vim.g.did_load_filetypes ~= 0 then
+  return
+end
+
+-- For now, make this opt-in with a global variable
+if vim.g.do_filetype_lua ~= 1 then
+  return
+end
+
+vim.cmd [[
+augroup filetypedetect
+au BufRead,BufNewFile * call v:lua.vim.filetype.match(str2nr(expand('<abuf>')))
+
+" These *must* be sourced after the autocommand above is created
+runtime! ftdetect/*.vim
+runtime! ftdetect/*.lua
+
+" Set a marker so that the ftdetect scripts are not sourced a second time by filetype.vim
+let g:did_load_ftdetect = 1
+
+augroup END
+]]