[ruby/erb] Make @scanner_map of ERB::Compiler::Scanner ractor-shareable

- Freeze on assignment
- Recreate Hash on registration

https://github.com/ruby/erb/commit/12d69fc2b3

lib/erb/compiler.rb

@@ -80,11 +80,17 @@ class ERB::Compiler # :nodoc:
   end
 
   class Scanner # :nodoc:
-    @scanner_map = {}
+    @scanner_map = defined?(Ractor) ? Ractor.make_shareable({}) : {}
     class << self
+      if defined?(Ractor)
+        def register_scanner(klass, trim_mode, percent)
+          @scanner_map = Ractor.make_shareable({ **@scanner_map, [trim_mode, percent] => klass })
+        end
+      else
         def register_scanner(klass, trim_mode, percent)
           @scanner_map[[trim_mode, percent]] = klass
         end
+      end
       alias :regist_scanner :register_scanner
     end