FEATURE: Serve RTL versions of admin and plugins CSS bundles for RTL locales (#21876)

Prior to this commit, we didn't have RTL versions of our admin and plugins CSS bundles and we always served LTR versions of those bundles even when users used an RTL locale, causing admin and plugins UI elements to never look as good as when an LTR locale was used. Example of UI issues prior to this commit were: missing margins, borders on the wrong side and buttons too close to each other etc.

This commit creates an RTL version for the admin CSS bundle as well as RTL bundles for all the installed plugins and serves those RTL bundles to users/sites who use RTL locales.

spec/lib/stylesheet/compiler_spec.rb

@@ -89,6 +89,20 @@ RSpec.describe Stylesheet::Compiler do
         expect(css).to include("background:")
       end
 
+      context "with the `rtl` option" do
+        it "generates an RTL version of the plugin CSS if the option is true" do
+          css, _ = Stylesheet::Compiler.compile_asset("scss_plugin", theme_id: theme.id, rtl: true)
+          expect(css).to include(".pull-left{float:right}")
+          expect(css).not_to include(".pull-left{float:left}")
+        end
+
+        it "returns an unchanged version of the plugin CSS" do
+          css, _ = Stylesheet::Compiler.compile_asset("scss_plugin", theme_id: theme.id, rtl: false)
+          expect(css).to include(".pull-left{float:left}")
+          expect(css).not_to include(".pull-left{float:right}")
+        end
+      end
+
       it "supports SCSS imports" do
         css, _map = Stylesheet::Compiler.compile_asset("scss_plugin", theme_id: theme.id)