provider: fix batchfile extension for ruby gem (neovim#7651)

ruby uses batchfiles with 'cmd' extension.
gem creates batchfiles with 'bat' extension.
`gem install rails` does the following in Windows (not Cygwin):

1. Run `gem.cmd install rails` on cmd.exe
2. gem.cmd runs `ruby.exe -x gem install rails`
3. `rails` gem is installed.
   `rails.bat` is created in the same directory
   where ruby.exe and gem.cmd reside.

runtime/autoload/provider/ruby.vim

@@ -19,7 +19,7 @@ function! provider#ruby#Detect() abort
   if exists("g:ruby_host_prog")
     return g:ruby_host_prog
   else
-    return has('win32') ? exepath('neovim-ruby-host.cmd') : exepath('neovim-ruby-host')
+    return has('win32') ? exepath('neovim-ruby-host.bat') : exepath('neovim-ruby-host')
   end
 endfunction