patch 9.0.1645: zserio files are not recognized

Problem:    zserio files are not recognized.
Solution:   Add a pattern for zserio files. (Dominique Pellé,
            closes #12544)

runtime/filetype.vim

@@ -1961,6 +1961,9 @@ au BufNewFile,BufRead .login,.cshrc,csh.cshrc,csh.login,csh.logout,*.csh,.alias
 au BufNewFile,BufRead *.zig			setf zig
 au BufNewFile,BufRead *.zir			setf zir
 
+" Zserio
+au BufNewFile,BufRead *.zs			setf zserio
+
 " Z-Shell script (patterns ending in a star further below)
 au BufNewFile,BufRead .zprofile,*/etc/zprofile,.zfbfmarks  setf zsh
 au BufNewFile,BufRead .zshrc,.zshenv,.zlogin,.zlogout,.zcompdump setf zsh

src/testdir/test_filetype.vim

@@ -801,6 +801,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     zimbu: ['file.zu'],
     zimbutempl: ['file.zut'],
     zir: ['file.zir'],
+    zserio: ['file.zs'],
     zsh: ['.zprofile', '/etc/zprofile', '.zfbfmarks', 'file.zsh',
           '.zcompdump', '.zlogin', '.zlogout', '.zshenv', '.zshrc',
           '.zcompdump-file', '.zlog', '.zlog-file', '.zsh', '.zsh-file',

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1645,
 /**/
     1644,
 /**/