Add more useful container checks

[daun]

Description

• Make debug plugin useful again 🤠 — prompted by docs update about known issues
• Warn about some common mistakes early on
  • Add check if containers are present initially
  • Add check if container selector matches multiple elements
  • Add check if any of the containers matches the animation selector
• Refactor checks into instance methods
• Tested in playground

Checks

• The PR is submitted to the master branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

[hirasso]

🚚💨