vim-patch:9.1.0971: filetype: SLNX files are not recognized

Problem:  filetype: SLNX files are not recognized
Solution: detect '*.slnx' files as xml filetype
          (Gustav Eikaas)

References:
https://blog.ndepend.com/slnx-the-new-net-solution-xml-file-format/
https://blog.jetbrains.com/dotnet/2024/10/04/support-for-slnx-solution-files/

closes: vim/vim#16334

3b3318b640

Co-authored-by: GustavEikaas <gustav.eikaas@gmail.com>

runtime/lua/vim/filetype.lua

@@ -1337,6 +1337,7 @@ local extension = {
   xlb = 'xml',
   xlc = 'xml',
   xba = 'xml',
+  slnx = 'xml',
   xpm = detect_line1('XPM2', 'xpm2', 'xpm'),
   xpm2 = 'xpm2',
   xqy = 'xquery',