vim-patch:9.0.0922: Mermaid files are not recognized (#21160)

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

364438d1e8

Co-authored-by: Bram Moolenaar <Bram@vim.org>

runtime/lua/vim/filetype.lua

@@ -659,6 +659,9 @@ local extension = {
   dmt = 'maxima',
   wxm = 'maxima',
   mel = 'mel',
+  mmd = 'mermaid',
+  mmdc = 'mermaid',
+  mermaid = 'mermaid',
   mf = 'mf',
   mgl = 'mgl',
   mgp = 'mgp',