Enhance spec:prepare command with tailwindcss:build command

jsbundling-rails (rails/jsbundling-rails#128)
and cssbundling-rails (rails/cssbundling-rails#116)
do the same and it's useful for people who are using RSpec.

Author: rmehner

lib/tasks/build.rake

@@ -22,6 +22,8 @@ Rake::Task["assets:precompile"].enhance(["tailwindcss:build"])
 
 if Rake::Task.task_defined?("test:prepare")
   Rake::Task["test:prepare"].enhance(["tailwindcss:build"])
+elsif Rake::Task.task_defined?("spec:prepare")
+  Rake::Task["spec:prepare"].enhance(["tailwindcss:build"])
 elsif Rake::Task.task_defined?("db:test:prepare")
   Rake::Task["db:test:prepare"].enhance(["tailwindcss:build"])
 end