FIX: Replace R2 gem with rtlcss for generating RTL CSS (#19636)

We've had a couple of problems with the R2 gem where it generated a broken RTL CSS bundle that caused a badly broken layout when Discourse is used in an RTL language, see a3ce93b and 5926386. For this reason, we're replacing R2 with `rtlcss` that can handle modern CSS features better than R2 does.

`rltcss` is written in JS and available as an npm package. Calling the `rltcss` from rubyland is done via the `rtlcss_wrapper` gem which contains a distributable copy of the `rtlcss` package and loads/calls it with Mini Racer. See https://github.com/discourse/rtlcss_wrapper for more details.

Internal topic: t/76263.

lib/stylesheet/compiler.rb

@@ -65,8 +65,8 @@ module Stylesheet
       result = engine.render
 
       if options[:rtl]
-        require "r2"
-        [R2.r2(result), nil]
+        require "rtlcss_wrapper"
+        [RtlcssWrapper.flip_css(result), nil]
       else
         source_map = engine.source_map
         source_map.force_encoding("UTF-8")