Add default comment style for Vue

This makes it so that the user does not need to configure
`commentstring` for Vue files to `<!-- %s -->` themselves with a
`FileType` autocmd.

Related to !14

Author: JoosepAlviste

lua/ts_context_commentstring/internal.lua

@@ -22,6 +22,7 @@ M.config = {
   php = '// %s',
   html = '<!-- %s -->',
   svelte = '<!-- %s -->',
+  vue = '<!-- %s -->',
 
   -- Languages that can have multiple types of comments
   tsx = {