Support template parameter packs in C++ lexer (rouge-ruby#1555)

The change in rouge-ruby#1495 to the way that the `template` keyword caused
template parameter packs not to be highlighted correctly. This commit
fixes that bug while also introducing a more robust set of rules for
highlighting identifiers succeeding the `template` keyword.

lib/rouge/lexers/cpp.rb

@@ -75,15 +75,16 @@ def self.reserved
         rule id, Name::Class, :pop!
 
         # template specification
-        rule %r/\s*(?=>)/m, Text, :pop!
-        rule %r/[.]{3}/, Operator
         mixin :whitespace
+        rule %r/[.]{3}/, Operator
+        rule %r/,/, Punctuation, :pop!
+        rule(//) { pop! }
       end
 
       state :template do
-        rule %r/>/, Punctuation, :pop!
+        rule %r/[>;]/, Punctuation, :pop!
         rule %r/typename\b/, Keyword, :classname
-        mixin :root
+        mixin :statements
       end
 
       state :case do

spec/visual/samples/cpp

@@ -93,6 +93,9 @@ template struct Z<double>;
 template<typename T> concept C1 = sizeof(T) != sizeof(int);
 template<C1 T> struct S1 { };
 template<C1 T> using Ptr = T*;
+template <typename, typename...>
+template<class...> struct Tuple { };
+template<typename ...Ts> void f(Ts...) {}
 
 // variadic template
 template<class F, class... Args>