Pre-RFC: Add `aria-current` attribute to Ember's `LinkTo` component

The `LinkTo` component lacks native support for the `aria-current` attribute.

From: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-current

> When you have a group of related elements, such as several links in a breadcrumb or steps in a multi-step flow, with one element in the group styled differently from the others to indicate to the sighted user that this is the current element within its group, the aria-current should be used to inform the assistive technology user what has been indicated via styling.

It seems that anytime `LinkTo` produces `class="active"` it should also append `aria-current="true"`

`aria-current="true"` is a minimum implementation though, ideally the other `aria-current` values (page, step, location, etc) should be supported. Perhaps with an additional attribute like so:

`Home`

which would render as

`Home`

This would be my first RFC for Ember, so any feedback would be appreciated.