Simplify scale registration

[benmccann]

This would make it much easier to register a scale. You only need to pass the scale and can pass multiple at once if you want I'm not sure I prefer this route, but thought it would be helpful for visualizing the differences between the approaches

@kurkle had suggested over Slack that as an alternative to #6979 to support tree shaking we create a separate entry point for npm such as index.es6.js and don't do the auto-registration.

Option 1

We might be able to use this register controllers as well, which would also be needed for tree shaking. I'm imagining you'd want to do something like:

import {registrationService, TimeScale, LinearScale, LineController} from 'chart.js';
registrationService.register(TimeScale, LinearScale, LineController);

Option 2

If you have auto-registration you wouldn't want to have an entry point that imports everything, so you'd want to import the modules, but wouldn't need to register, so it'd end up looking like:

import TimeScale from 'chart.js/scale.time';
import LinearScale from 'chart.js/scale.linear';
import LineController from 'chart.js/controller.line';

Option 3

Another option would be to remove the registration and pass the constructor type directly in the opions:

import TimeScale from 'chart.js/scale.time';
const options = {
  scales: {
    x: {
      type: TimeScale
    }
  }
};

[benmccann]

Closing for now. I may revisit later