Deprecate Lexer::Elem#[] and Lexer::State#[]

Discussed in https://github.com/ruby/ruby/pull/5093#issuecomment-964426481. 

> it would be enough to mimic only [] for almost all cases

This adds back the `Lexer::Elem#[]` and `Lexer::State#[]` and adds deprecation warnings for them.

ext/ripper/lib/ripper/lexer.rb

@@ -62,6 +62,17 @@ class Ripper
         freeze
       end
 
+      def [](index)
+        case index
+        when 0
+          warn "Calling `Lexer::State#[0]` is deprecated, please use `Lexer::State#to_int` instead"
+          @to_int
+        when 1
+          warn "Calling `Lexer::State#[1]` is deprecated, please use `Lexer::State#to_s` instead"
+          @event
+        end
+      end
+
       alias to_i to_int
       alias inspect to_s
       def pretty_print(q) q.text(to_s) end
@@ -84,6 +95,26 @@ class Ripper
         @message = message
       end
 
+      def [](index)
+        case index
+        when 0
+          warn "Calling `Lexer::Elem#[0]` is deprecated, please use `Lexer::Elem#pos` instead"
+          @pos
+        when 1
+          warn "Calling `Lexer::Elem#[1]` is deprecated, please use `Lexer::Elem#event` instead"
+          @event
+        when 2
+          warn "Calling `Lexer::Elem#[2]` is deprecated, please use `Lexer::Elem#tok` instead"
+          @tok
+        when 3
+          warn "Calling `Lexer::Elem#[3]` is deprecated, please use `Lexer::Elem#state` instead"
+          @state
+        when 4
+          warn "Calling `Lexer::Elem#[4]` is deprecated, please use `Lexer::Elem#message` instead"
+          @message
+        end
+      end
+
       def inspect
         "#<#{self.class}: #{event}@#{pos[0]}:#{pos[1]}:#{state}: #{tok.inspect}#{": " if message}#{message}>"
       end