Set up CI test matrix to test three-elements against different versions of Three.js

[hmans]

We should test against a bunch of recent Three.js releases to be able to quickly spot regressions if there are any.

• See ci.yml
• See Using a Build Matrix
• Now we just need to figure out how to inject the different three.js versions into our package.json. :S

[CodyJasonBennett]

Is there a cut-off point for three releases we should test against? I'm worried about slowed tests.

[hmans]

Is there a cut-off point for three releases we should test against? I'm worried about slowed tests.

I would set it up to test against maybe the last 3 major releases. Also, consider that this is only in GitHub Actions; locally, tests will only run against whatever we have configured in devDependencies (which will typically be the newest release.)