Support mustache template in Svelte

This ensures we delegate the handling of `{{}}` in mustache template to
JS lexer.

lib/rouge/lexers/svelte.rb

@@ -62,7 +62,7 @@ def initialize(*)
         rule %r/}/, Str::Interpol, :pop!
 
         # Allow JS lexer to handle matched curly braces within template
-        rule(/(?<=^|[^\\])\{.*?(?<=^|[^\\])\}/) do
+        rule(/(?<=^|[^\\])\{+.*?(?<=^|[^\\])\}+/) do
           delegate @js
         end