Add Link component

[mkernohanbc]

WIP/draft PR for discussion and feedback. @Supriya-Arora: this is carved out from a bigger piece of work around menus/navbars. Also cc'ing @ty2k for input — we've discussed this a little bit, would appreciate your take.

This PR adds a new Link component, based on RAC Link. Docs and examples are included in Storybook.

As the name suggests, Link is a utility component for creating accessible and extensible hyperlinks. In the design system context, its primary usage is in navigation (eg. populating menus within the Header and Footer components), but it can also be used inline.

Text links

Link text is populated using the children slot. Link behaviour is configured using the href (renders a standard <a> element) or onPress (renders a <span> with the ARIA "link" role) props. See the RAC docs for information about events and routing.

A Link renders with the .bcds-react-aria-Link class. The size and danger props are used to append additional classes (toggling between S/M/L text sizes and a red colourway for destructive or dangerous links.)

The user can override link styling entirely by either:

• Passing their own CSS class using className
• Using the isUnstyled prop, which removes all styling and allows a link to inherit its styles from its parents

A Link can be disabled using isDisabled.

Link buttons

Using the isButton prop, Link can also be styled to look and behave like the Button component. When isButton is set, Link will render with the .bcds-react-aria-Button class instead of .bcds-react-aria-Link. The buttonVariant prop replicates the functionality of variant on the Button component. Link imports Button.css directly to avoid duplicating styles.

The reasoning for this functionality is to better align with how RAC Button works:

The Button component always represents a button semantically. To create a link that visually looks like a button, use the Link component instead.

Unlike our Button component, Link supports the href prop natively.

[Supriya-Arora]

Looks good to me.