[EphemeralPresenceManager] Add support for hovercard presence (#11554)

### WHY are these changes introduced?

This PR unblocks
[HoverCard](#10504) from tracking
presence using the `useEphemeralPresenceManager` hook. The component and
hook logic are both agnostic to the actual list of supported keys
defined in the default state object, so no additional tests are needed.

Author: chloerice

.changeset/nervous-laws-sparkle.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Added support for hovercard tracking to `EphemeralPresenceManager`

polaris-react/src/components/EphemeralPresenceManager/EphemeralPresenceManager.tsx

@@ -20,6 +20,7 @@ type PresenceCounter = {
 
 const defaultState = {
   tooltip: 0,
+  hovercard: 0,
 };
 
 export function EphemeralPresenceManager({