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: #16334

Signed-off-by: GustavEikaas <gustav.eikaas@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/filetype.vim

@@ -2414,6 +2414,7 @@ au BufNewFile,BufRead *.ice			setf slice
 " Microsoft Visual Studio Solution
 au BufNewFile,BufRead *.sln			setf solution
 au BufNewFile,BufRead *.slnf			setf json
+au BufNewFile,BufRead *.slnx			setf xml
 
 " Spice
 au BufNewFile,BufRead *.sp,*.spice		setf spice

src/testdir/test_filetype.vim

@@ -892,7 +892,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     xml: ['/etc/blkid.tab', '/etc/blkid.tab.old', 'file.xmi', 'file.csproj', 'file.csproj.user', 'file.fsproj', 'file.fsproj.user', 'file.vbproj', 'file.vbproj.user', 'file.ui',
           'file.tpm', '/etc/xdg/menus/file.menu', 'fglrxrc', 'file.xlf', 'file.xliff', 'file.xul', 'file.wsdl', 'file.wpl', 'any/etc/blkid.tab', 'any/etc/blkid.tab.old',
           'any/etc/xdg/menus/file.menu', 'file.atom', 'file.rss', 'file.cdxml', 'file.psc1', 'file.mpd', 'fonts.conf', 'file.xcu', 'file.xlb', 'file.xlc', 'file.xba', 'file.xpr',
-          'file.xpfm', 'file.spfm', 'file.bxml', 'file.mmi'],
+          'file.xpfm', 'file.spfm', 'file.bxml', 'file.mmi', 'file.slnx'],
     xmodmap: ['anyXmodmap', 'Xmodmap', 'some-Xmodmap', 'some-xmodmap', 'some-xmodmap-file', 'xmodmap', 'xmodmap-file'],
     xpm: ['file.xpm'],
     xpm2: ['file.xpm2'],

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    971,
 /**/
     970,
 /**/