patch 9.0.0922: Mermaid files are not recognized

Problem:    Mermaid files are not recognized.
Solution:   Add patterns for Mermaid. (Crag MacEachern)

src/testdir/test_filetype.vim

@@ -349,6 +349,7 @@ let s:filename_checks = {
     \ 'maxima': ['file.demo', 'file.dmt', 'file.dm1', 'file.dm2', 'file.dm3',
     \            'file.wxm', 'maxima-init.mac'],
     \ 'mel': ['file.mel'],
+    \ 'mermaid': ['file.mmd', 'file.mmdc', 'file.mermaid'],
     \ 'meson': ['meson.build', 'meson_options.txt'],
     \ 'messages': ['/log/auth', '/log/cron', '/log/daemon', '/log/debug', '/log/kern', '/log/lpr', '/log/mail', '/log/messages', '/log/news/news', '/log/syslog', '/log/user',
     \     '/log/auth.log', '/log/cron.log', '/log/daemon.log', '/log/debug.log', '/log/kern.log', '/log/lpr.log', '/log/mail.log', '/log/messages.log', '/log/news/news.log', '/log/syslog.log', '/log/user.log',