Feature request: d2l-button-link component

[dlockhart]

While rare, there are certain scenarios where it makes sense for something to appear visually as a button but semantically be a link.

Here's one example:

Using semantic links (backed by the <a> element) have the advantage of appearing as a link to assistive technologies like screen readers and allowing users to right-click and open in a new tab (or Ctrl/Cmd-click it to do the same).

To accomplish this, we should introduce a <d2l-button-link> component.

The component will:

• Take href and target attributes similar to d2l-link
• Support the primary attribute to style it like a primary button
• Take a text attribute for the stuff to put inside the <a> element instead of using a slot like <d2l-button> and <d2l-link> do. This gives us more control over.
• Not extend the ButtonMixin like the other button-like components, since we don't want it to have button-only attributes like disabled, form, type, etc.
• Expose and use buttonLinkStyles (similar to button-styles.js) such that the styles could just be applied directly to an <a class="d2l-button-link"> element
• Support the FocusVisiblePolyfillMixin
• Be a supported child of the <d2l-overflow-group> component
• Have visual diff tests, unit tests, accessibility tests
• Be included in the existing button documentation