Improve optional chaining in JavaScript lexer (#1594)

Previously, the example `${obj?.prop}` would result in a runaway string
literal, since the `?` would start a ternary expression, and Rouge
would lose the stack state causing the second `` ` `` to terminate the
literal. This commit fixes that issue.

lib/rouge/lexers/javascript.rb

@@ -162,6 +162,10 @@ def self.id_regex
           push :template_string
         end
 
+        # special case for the safe navigation operator ?.
+        # so that we don't start detecting a ternary expr
+        rule %r/[?][.]/, Punctuation
+
         rule %r/[?]/ do
           token Punctuation
           push :ternary

spec/visual/samples/javascript

@@ -229,6 +229,8 @@ function* range(from, to)
   1 + 2
 }`;
 
+var notATernary = `${obj?.prop}`;
+
 @(function(thing) { return thing; })
 class Person {
   @deprecate