Add ignored_sp event

	* ext/ripper/lib/ripper/lexer.rb (Ripper::Lexer): add ignored_sp
	  event which will be fired from Ripper::Lexer#on_heredoc_dedent
	  method.  [ruby-core:91727] [Bug #15648]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67208 b2dd03c8-39d4-4d8f-98ff-823fe69b080e