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',

test/old/testdir/test_filetype.vim

@@ -287,6 +287,7 @@ func s:GetFilenameChecks() abort
     \ 'fgl': ['file.4gl', 'file.4gh', 'file.m4gl'],
     \ 'firrtl': ['file.fir'],
     \ 'fish': ['file.fish'],
+    \ 'flix': ['file.flix'],
     \ 'focexec': ['file.fex', 'file.focexec'],
     \ 'form': ['file.frm'],
     \ 'forth': ['file.ft', 'file.fth', 'file.4th'],