Allow non-memoized item lists

Towards primer#4315

The SelectPanel only did a basic equality check for the item state, meaning
that it depended on having the exact same objects on multiple pass throughs.

This isn't always possible as sometimes you may want to have different objects.

This replaces the equality check with a test for an `id` property on the object.

If the `id` property isn't present, we fallback to the old behavior.

Note that a previous version used the `key` prop, but we decided `id` was a
better interface. f472da2#r139163795
Co-authored-by: Andrew Henry <[email protected]>

.changeset/violet-yaks-lie.md

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+---
+'@primer/react': patch
+---
+
+Fixes a bug in `SelectPanel` when the list of items was not memoized.