Make the formatter mechanism support Ractor

Now the formatter configuration is per Ractor. DefaultFormatter is used
if not set.

DefaultFormatter#message_for is now a class method to allow sub-Ractors
to call the method.

lib/error_highlight/formatter.rb

@@ -1,6 +1,6 @@
 module ErrorHighlight
   class DefaultFormatter
-    def message_for(spot)
+    def self.message_for(spot)
       # currently only a one-line code snippet is supported
       if spot[:first_lineno] == spot[:last_lineno]
         indent = spot[:snippet][0...spot[:first_column]].gsub(/[^\t]/, " ")
@@ -14,12 +14,10 @@ def message_for(spot)
   end
 
   def self.formatter
-    @@formatter
+    Ractor.current[:__error_highlight_formatter__] || DefaultFormatter
   end
 
   def self.formatter=(formatter)
-    @@formatter = formatter
+    Ractor.current[:__error_highlight_formatter__] = formatter
   end
-
-  self.formatter = DefaultFormatter.new
 end

test/test_error_highlight.rb

@@ -5,15 +5,15 @@
 
 class ErrorHighlightTest < Test::Unit::TestCase
   class DummyFormatter
-    def message_for(corrections)
+    def self.message_for(corrections)
       ""
     end
   end
 
   def setup
     if defined?(DidYouMean)
       @did_you_mean_old_formatter = DidYouMean.formatter
-      DidYouMean.formatter = DummyFormatter.new
+      DidYouMean.formatter = DummyFormatter
     end
   end