consistently use ::JSON.<method> in ext.rb

Author: GrantBirki

lib/json/ext.rb

@@ -30,14 +30,14 @@ def parse
 
     require 'json/ext/parser'
     Ext::Parser::Config = Ext::ParserConfig
-    JSON.parser = Ext::Parser
+    ::JSON.parser = Ext::Parser
 
     if RUBY_ENGINE == 'truffleruby'
       require 'json/truffle_ruby/generator'
-      JSON.generator = ::JSON::TruffleRuby::Generator
+      ::JSON.generator = ::JSON::TruffleRuby::Generator
     else
       require 'json/ext/generator'
-      JSON.generator = Generator
+      ::JSON.generator = Generator
     end
   end