patch 8.2.3880: Solution filter files are not recognized

Problem:    Solution filter files are not recognized.
Solution:   Add pattern *.slnf and use json. (Doug Kearns)

runtime/filetype.vim

@@ -1750,6 +1750,7 @@ au BufNewFile,BufRead *.ice			setf slice
 
 " Microsoft Visual Studio Solution
 au BufNewFile,BufRead *.sln			setf solution
+au BufNewFile,BufRead *.slnf			setf json
 
 " Spice
 au BufNewFile,BufRead *.sp,*.spice		setf spice

src/testdir/test_filetype.vim

@@ -263,7 +263,7 @@ let s:filename_checks = {
     \ 'jgraph': ['file.jgr'],
     \ 'jovial': ['file.jov', 'file.j73', 'file.jovial'],
     \ 'jproperties': ['file.properties', 'file.properties_xx', 'file.properties_xx_xx', 'some.properties_xx_xx_file'],
-    \ 'json': ['file.json', 'file.jsonp', 'file.json-patch', 'file.webmanifest', 'Pipfile.lock', 'file.ipynb', '.babelrc', '.eslintrc', '.prettierrc', '.firebaserc'],
+    \ 'json': ['file.json', 'file.jsonp', 'file.json-patch', 'file.webmanifest', 'Pipfile.lock', 'file.ipynb', '.babelrc', '.eslintrc', '.prettierrc', '.firebaserc', 'file.slnf'],
     \ 'jsonc': ['file.jsonc'],
     \ 'jsp': ['file.jsp'],
     \ 'julia': ['file.jl'],

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3880,
 /**/
     3879,
 /**/