vim-patch:9.1.0187: filetype: no support for Dafny files (#27918)

Problem:  Dafny files are not recognized.
Solution: Recognize *.dfy files as filetype "dafny" (zeertzjq).

Ref: https://dafny.org/
Ref: https://github.com/mlr-msft/vim-loves-dafny

closes: vim/vim#14226

4e334d0443

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

runtime/lua/vim/filetype.lua

@@ -335,6 +335,7 @@ local extension = {
   si = 'cuplsim',
   cyn = 'cynpp',
   cypher = 'cypher',
+  dfy = 'dafny',
   dart = 'dart',
   drt = 'dart',
   ds = 'datascript',

test/old/testdir/test_filetype.vim

@@ -179,6 +179,7 @@ func s:GetFilenameChecks() abort
     \ 'cynpp': ['file.cyn'],
     \ 'cypher': ['file.cypher'],
     \ 'd': ['file.d'],
+    \ 'dafny': ['file.dfy'],
     \ 'dart': ['file.dart', 'file.drt'],
     \ 'datascript': ['file.ds'],
     \ 'dcd': ['file.dcd'],