Use polymorphic slots in ActionList and NavList

[camertron]

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What are you trying to accomplish?

This PR removes a couple of hacks in ActionList and NavList that call a private view_component API method (i.e. #set_slot) in order to fill in a slot with heterogeneous objects. The view_component framework offers a polymorphic slots feature tailor-made for such a use-case, but we haven't been able to make use of it... until now! I recently landed a feature that allows customization of polymorphic slot setter methods, which allows us to maintain backwards-compatibility with the existing public APIs of the two components in question and stop calling that private VC method.

NavList and ActionList both manage a list of items. These items can be regular 'ol items, dividers, or groups of items. Before the advent of dividers and groups, the public APIs of these components only offered a #with_item method via the items slot. Converting the items slot into a polymorphic one would have caused the #with_item method to become something like eg. #with_list_item, while dividers could have been added via #with_item_divider, etc. This is because slot setter methods were automatically generated and did not allow customization. As of view_component v3.1.0 however, slot setters can be completely customized.

Integration

Yes. Dotcom will need to be upgraded to use view_component v3.1.0. This should be very little trouble however, since 3.1.0 is only a single minor version away from the version dotcom is currently using.

List the issues that this change affects.

Closes #2041

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

Merge checklist

• Added/updated documentation

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🦋 Changeset detected

Latest commit: 72cc313

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Minor

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