Add snippet for # frozen_string_literal: true

See https://wyeworks.com/blog/2015/12/1/immutable-strings-in-ruby-2-dot-3 for a detailed description of the frozen_string_literal comment.

snippets/ruby.snippets

@@ -1,5 +1,7 @@
 snippet enc
 	# encoding: utf-8
+snippet frozen
+	# frozen_string_literal: true
 snippet #!
 	#!/usr/bin/env ruby
 # New Block