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>
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 521c611..3c3bde2 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2025 Mar 27
+*syntax.txt*	For Vim version 9.1.  Last change: 2025 Apr 13
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2451,6 +2451,12 @@
 
 	let filetype_m = "mma"
 
+MBSYNC					*mbsync.vim* *ft-mbsync-syntax*
+
+The mbsync application uses a configuration file to setup mailboxes names,
+user and password. All files ending with `.mbsyncrc` or with the name
+`isyncrc` will be recognized as mbsync configuration files.
+
 MEDIAWIKI					*ft-mediawiki-syntax*
 
 By default, syntax highlighting includes basic HTML tags like style and
diff --git a/runtime/doc/tags b/runtime/doc/tags
index e0006da..6b27199 100644
--- a/runtime/doc/tags
+++ b/runtime/doc/tags
@@ -7430,6 +7430,7 @@
 ft-masm-syntax	syntax.txt	/*ft-masm-syntax*
 ft-mathematica-syntax	syntax.txt	/*ft-mathematica-syntax*
 ft-matlab-indent	indent.txt	/*ft-matlab-indent*
+ft-mbsync-syntax	syntax.txt	/*ft-mbsync-syntax*
 ft-mediawiki-syntax	syntax.txt	/*ft-mediawiki-syntax*
 ft-metafont	ft_mp.txt	/*ft-metafont*
 ft-metafont-intro	ft_mp.txt	/*ft-metafont-intro*
@@ -8854,6 +8855,7 @@
 matlab-indenting	indent.txt	/*matlab-indenting*
 max()	builtin.txt	/*max()*
 maxcol-variable	eval.txt	/*maxcol-variable*
+mbsync.vim	syntax.txt	/*mbsync.vim*
 mbyte-IME	mbyte.txt	/*mbyte-IME*
 mbyte-XIM	mbyte.txt	/*mbyte-XIM*
 mbyte-combining	mbyte.txt	/*mbyte-combining*