Not compatible with Chart.js v3

When I upgrade Chart.js from 2.x to 3.0.2, I get:

```
Chart.PieceLabel.js:409 Uncaught TypeError: Cannot read property 'register' of undefined
    at Chart.PieceLabel.js:409
    at Chart.PieceLabel.js:418
```


It seems that the way plugins are registered has changed:
https://www.chartjs.org/docs/3.0.2/developers/plugins.html