vim-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: vim/vim#17103

836b87d699

Co-authored-by: Pierrick Guillaume <pguillaume@fymyte.com>

runtime/ftplugin/mbsync.vim

@@ -0,0 +1,13 @@
+" Vim filetype plugin file
+" Language:		mbsync configuration file
+" Maintainer:		Pierrick Guillaume <pguillaume@fymyte.com>
+" Last Change:		2025 Apr 13
+
+if (exists('b:did_ftplugin'))
+  finish
+endif
+let b:did_ftplugin = 1
+
+let b:undo_ftplugin = "setlocal commentstring<"
+
+setlocal commentstring=#\ %s