patch 9.1.1299: filetype: mbsyncrc files are not recognized

Problem:  filetype: mbsyncrc files are not recognized
Solution: detect isyncrc and "*.mbsyncrc" files as mbsync filetype,
          include filetype and syntax plugin (Pierrick Guillaume)

mbsync is a command line application which synchronizes mailboxes;
currently Maildir and IMAP4 mailboxes are supported.
New messages, message deletions and flag changes can be propagated both ways;
the operation set can be selected in a fine-grained manner.

References:
mbsync syntax overview: mbsync manual (isync v1.4.4)
https://isync.sourceforge.io/mbsync.html

Upstream support for the mbsync filetype.
Original plugin: https://github.com/Fymyte/mbsync.vim

closes: #17103

Signed-off-by: Pierrick Guillaume <pguillaume@fymyte.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_filetype.vim

@@ -181,7 +181,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     cobol: ['file.cbl', 'file.cob'],
     coco: ['file.atg'],
     conaryrecipe: ['file.recipe'],
-    conf: ['auto.master', 'file.conf', 'texdoc.cnf', '.x11vncrc', '.chktexrc', '.ripgreprc', 'ripgreprc', 'file.ctags', '.mbsyncrc'],
+    conf: ['auto.master', 'file.conf', 'texdoc.cnf', '.x11vncrc', '.chktexrc', '.ripgreprc', 'ripgreprc', 'file.ctags'],
     config: ['configure.in', 'configure.ac', '/etc/hostname.file', 'any/etc/hostname.file'],
     confini: ['pacman.conf', 'paru.conf', 'mpv.conf', 'any/.aws/config', 'any/.aws/credentials', 'file.nmconnection'],
     context: ['tex/context/any/file.tex', 'file.mkii', 'file.mkiv', 'file.mkvi', 'file.mkxl', 'file.mklx'],
@@ -472,6 +472,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     matlab: ['file.m'],
     maxima: ['file.demo', 'file.dmt', 'file.dm1', 'file.dm2', 'file.dm3',
                 'file.wxm', 'maxima-init.mac'],
+    mbsync: ['.mbsyncrc', 'file.mbsyncrc', 'isyncrc'],
     mediawiki: ['file.mw',  'file.wiki'],
     mel: ['file.mel'],
     mermaid: ['file.mmd', 'file.mmdc', 'file.mermaid'],

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1299,
 /**/
     1298,
 /**/