To work toward exposing something like the get...Items functions.

optimizedEvaluateItems falls back to evaluating all items for unsorted items, and sorting / optimizing ought to be okay, so this ought to be equivalent.

helpers.dom.js's getRelativePosition already had logic to handle ChartEvent vs. Event (as demonstrated by the `native` check within `getCanvasPosition`), so it's redundant for core.interaction.js to have its own `native` check.

Update `getRelativePosition` to use the same `native` check as core.interaction.js's version.  As best as I can tell, the ChartEvent's x and y are populated from `getRelativePosition`, so the previous `getCanvasPosition` was effectively just duplicating `getRelativePosition'`s work.  I added a test to verify this; it depends on a local, not-yet-submitted change in chartjs-test-utils' `triggerMouseEvent` to return the mouse event that it triggers.

Add a non-null assertion, as requested in code review.

Add JSDoc to clarify that `getCanvasPosition` now expects a native `Event`, not a `ChartEvent`.  Add `@ts-ignore`; `getCanvasPosition` relied on some loose conversions between `Event`, `TouchEvent`, and `Touch` that would require several changes to make TypeScript happy.

Return the event directly, to speed up the code a bit.  Add JSDoc to help communicate its intent.  Update various comments.

Only thing needed actually is the update of chartjs-test-utils to 0.4.0

To get supporting work from chartjs/chartjs-test-utils#19