routerLinkActive not supported by anchor tabs and breadcrumb

🐛 Bug Report

In Angular, routerLinkActive is an attribute used to help style an active link element, e.g. the active anchor tab, or the breadcrumb item for the current page. Currently, we use different schemes to style the active item in each of those components. We need to consider integration with the routerLinkActive attribute.

💻 Repro or Code Sample

<nimble-anchor-tabs activeid="1">
                <nimble-anchor-tab id="1" [nimbleRouterLink]="url1" routerLinkActive="active">Tab 1</nimble-anchor-tab>
                <nimble-anchor-tab id="2" [nimbleRouterLink]="url2" routerLinkActive="active">Tab 2</nimble-anchor-tab>
</nimble-anchor-tabs>

🤔 Expected Behavior

When the user activates a tab by clicking or Space/Enter, we should have a way to set the nimble-anchor-tabs activeid to the id of that tab. This could be communicated via an example program, or by a custom directive (e.g. nimbleRouterLinkActive) that encapsulates that logic.

We need to investigate if/how the breadcrumb needs to support routerLinkActive in its own way, e.g. clearing href on active breadcrumb item.

😯 Current Behavior

Anchor tabs and breadcrumb don't care about routerLinkActive.

💁 Possible Solution

Anchor tabs: Set activeid of tabs based on isActive property of RouterLinkActive directives of child anchor tab elements.

🔦 Context

Came up in discussion on PR #987

Start with the anchor tabs and breadcrumb components described in the issue, then read Angular's RouterLinkActive documentation and the discussion in PR #987. Determine how active router links should update tab activeid and whether the breadcrumb needs separate handling; done means both components have an agreed, demonstrable integration path.