feat(#222): checks if required assets were built in pull requests

[pedroqueiroga]

• Names test step
• Creates new step named Build pyp5js
  • python3 setup.py sdist bdist_wheel
• Creates new step named Check required files
  • runs new bash script bin/check_required_assets.sh
• Creates new bash script bin/check_required_assets.sh
  • reads from bin/check_required_assets.txt file
  • sanitizes any semver it finds
  • glob searches for the assets, expanding semvers

Should we check every single ace asset built? I'm checking only for ace.js and expecting the rest to be there if it is, so that if ace.js decides to package new files or rename them it won't break builds.

Closes #222