vim-patch:9.1.1878: filetype: Soy files are not recognized (#36346)

Problem:  filetype: Soy files are not recognized
Solution: Detect *.soy files as soy filetype,
          include a soy filetype plugin (Riley Bruins)

closes: vim/vim#18640

9fff99c3c2

Co-authored-by: Riley Bruins <ribru17@hotmail.com>

test/old/testdir/test_filetype.vim

@@ -744,6 +744,7 @@ func s:GetFilenameChecks() abort
     \ 'snobol4': ['file.sno', 'file.spt'],
     \ 'solidity': ['file.sol'],
     \ 'solution': ['file.sln'],
+    \ 'soy': ['file.soy'],
     \ 'spajson': ['any/pipewire/file.conf', 'any/pipewire/file.conf.d/other.conf',
     \             'any/wireplumber/file.conf', 'any/wireplumber/file.conf.d/other.conf'],
     \ 'sparql': ['file.rq', 'file.sparql'],