feat: filetype.lua (#16600)

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

src/nvim/lua/vim.lua

@@ -40,6 +40,9 @@ assert(vim)
 vim.inspect = package.loaded['vim.inspect']
 assert(vim.inspect)
 
+vim.filetype = package.loaded['vim.filetype']
+assert(vim.filetype)
+
 local pathtrails = {}
 vim._so_trails = {}
 for s in  (package.cpath..';'):gmatch('[^;]*;') do