Add countLabel prop to SearchListItem

[Aljullu]

In the Reviews by Product block we need to display X Reviews next to each product in the SearchListControl:

Currently, SearchListItem only allows showing a number (which comes from item.count). This PR adds a countLabel prop to SearchListItem that will overwrite what's rendered in the counter. Allowing the consumer to modify it.

Accessibility

• I've tested using only a keyboard (no mouse)
• I've tested using a screen reader
• All text has at least a 4.5 color contrast with its background

Screenshots

Detailed test instructions:

• Modify this line:
  https://github.com/woocommerce/woocommerce-admin/blob/master/packages/components/src/search-list-control/index.js#L107
  With this or something similar:

		return <SearchListItem countLabel="Lorem Ipsum" showCount { ...args } />;

• Go to WooCommerce > DevDocs.
• Go to SearchListControl card and press Toggle loading state.
• Verify the count label appears next to the list names (see screenshot above).

[mikejolley]

This works great. @timmyc There are a few instances in blocks that could use this (featured variations is one), as well as the new reviews block that is currently blocked by this. Would it be possible to get this in the package so we can use it?

[timmyc]

Would it be possible to get this in the package so we can use it?

Indeed lets merge this and release a new version of components so you all can consume it in blocks 👍