patch 9.0.1503: Luau files are not recognized

Problem:    Luau files are not recognized.
Solution:   Add a patter for Luau files. (Amaan Qureshi, closes #12317)

runtime/filetype.vim

@@ -1190,6 +1190,9 @@ au BufNewFile,BufRead *.lou,*.lout		setf lout
 " Lua
 au BufNewFile,BufRead *.lua			setf lua
 
+" Luau
+au BufNewFile,BufRead *.luau		setf luau
+
 " Luacheck
 au BufNewFile,BufRead .luacheckrc		setf lua
 

src/testdir/test_filetype.vim

@@ -346,6 +346,7 @@ let s:filename_checks = {
     \ 'lsl': ['file.lsl'],
     \ 'lss': ['file.lss'],
     \ 'lua': ['file.lua', 'file.rockspec', 'file.nse', '.luacheckrc', '.busted'],
+    \ 'luau': ['file.luau'],
     \ 'lynx': ['lynx.cfg'],
     \ 'lyrics': ['file.lrc'],
     \ 'm3build': ['m3makefile', 'm3overrides'],

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1503,
 /**/
     1502,
 /**/