vim-patch:9.1.1501: filetype: flix files are not recognized

Problem:  filetype: flix files are not recognized
Solution: detect *.flix files as flix filetype
          (0xadk)

References:
- https://flix.dev/
- https://doc.flix.dev/introduction.html

closes: vim/vim#17646

b211916e0a

Co-authored-by: 0xadk <0xadk@users.noreply.github.com>

runtime/lua/vim/filetype.lua

@@ -464,6 +464,7 @@ local extension = {
   ['4gh'] = 'fgl',
   fir = 'firrtl',
   fish = 'fish',
+  flix = 'flix',
   focexec = 'focexec',
   fex = 'focexec',
   ft = 'forth',