patch 9.1.1878: filetype: Soy files are not recognized

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

closes: #18640

Signed-off-by: Riley Bruins <ribru17@hotmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_filetype.vim

@@ -747,6 +747,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     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'],

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1878,
 /**/
     1877,
 /**/