Event Definition Variables Support

[pinussilvestrus]

• migrate CI to GitHub actions ⚙️
• adds support for
  • camunda:errorMessageVariable
  • camunda:errorCodeVariable
  • camunda:escalationCodeVariable
• adds some documentation to make things clearer

Which issue does this PR address?

Closes #9

Acceptance Criteria

• [ ] Corresponds to the concept
• [ ] Corresponds to the design

Definition of Done

• [ ] corresponds to the design principles
• [ ] corresponds to the code standards
• [ ] passes Continuous Integration checks
• [ ] available as feature branch on GitHub
  • [ ] contains the cleaned up commit history
  • [ ] commit messages satisfy our commit message guidelines
  • [ ] a single commit closes the issue via Closes #issuenr

[MaxTru]

Not blocking this PR, but we should toggle it as part of https://github.com/camunda/camunda-modeler/issues/2070: you can define twice the same variable in a single event:

[nikku]

Not blocking this PR, but we should toggle it as part of camunda/camunda-modeler#2070: you can define twice the same variable in a single event.

Not a feature request in this library, right?

[MaxTru]

Oops, I was too late. I think the empty string "issue" is worth a thought.

[pinussilvestrus]

No problem. Let's discuss the open issues and I'll follow up with a second PR.

[nikku]

My bad, was too quick with my fingers. @pinussilvestrus I guess Max thoughts are worth another look?