error messages on unsupported platforms or when bundler platforms aren't correct

[flavorjones]

what problem is being solved

See #101 and comments in #96 to see some problems users are having where either:

• the ruby platform (vanilla) gem is installed and so tailwindcss is simply not found in the user's path
• bundler's lockfile needs to be updated with the current platform

what changes are being made

This PR introduces two changes:

• upstream tailwindcss metadata are moved from the rakefile into lib/tailwindcss/upstream.rb for use by the gem
• the exe/tailwindcss script is always shipped with the gem

An job is added to the gem-install actions pipeline to test the scenario where a bundler lockfile needs to have the native platform added.

what will users see

The net result is that when someone has the "ruby" platform vanilla gem installed because their platform is not supported, they will see this error message when they run "tailwindcss":

ERROR: tailwindcss-rails does not support the <platformname> platform
Please install tailwindcss following instructions at https://tailwindcss.com/docs/installation

When someone has the "ruby" platform vanilla gem installed because their bundler lockfile does not contain the current system's platform, they will see this error message when they run "tailwindcss":

ERROR: Cannot find the tailwindcss executable for x86_64-linux in /home/flavorjones/.rbenv/versions/3.0.3/lib/ruby/gems/3.0.0/gems/tailwindcss-rails-2.0.0/exe
If you're using bundler, please make sure you're on the latest bundler version:

  gem install bundler
  bundle update --bundler

Then make sure your lock file includes this platform by running:

  bundle lock --add-platform x86_64-linux
  bundle install

See `bundle lock --help` output for details.