Add Racc as a development dependency (#747)

st0012 · web-flow · commit 3a28eeea4dfc · 2023-11-06T22:29:06.000+09:00
In ruby/rdoc#1019 RDoc starts embedding Racc. But Racc in Ruby 3.3 will not be a default gem anymore, which requires explicit declaration in Gemfile.
diff --git a/Gemfile b/Gemfile
@@ -17,3 +17,5 @@ gem "rake"
 gem "test-unit"
 gem "test-unit-ruby-core"
 gem "debug", github: "ruby/debug"
+
+gem "racc"
