Description

Makes JointDef abstract.

Checklist

[X] The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
[X] I have read the Contributor Guide and followed the process outlined for submitting PRs.
[X] I have updated/added tests for ALL new/updated/fixed functionality.
[X] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
[X] I have updated/added relevant examples in examples.