Add kirbyXComboboxItemTemplate directive with clear documentation

[HootERasmus]

Which issue does this PR close?

This PR closes #4518

What is the new behavior?

Added kirbyXComboboxItemTemplate directive to enable custom item templating in the Combobox component. This directive provides context variables (item, selected, focused) for templated items and must be applied to the kirby-item element when using custom templates.

The directive follows the naming convention of prefixing with 'X' (like kirby-x-combobox), and comprehensive storybook documentation has been added to make it clear when and how to use this directive.

Does this PR introduce a breaking change?

• Yes
• No

Key Changes

• Renamed template selector to kirbyXComboboxItemTemplate for consistency
• Updated storybook stories (WithTemplate, CustomSearchFunction, LargeList) to demonstrate the directive usage
• Added prominent documentation notes in all affected stories explaining the importance of the directive
• Enhanced JSDoc comments to clarify that this directive is required for custom item templates

Checklist:

The following tasks should be carried out in sequence in order to follow the process of contributing correctly.

Reminders

• Make sure you have implemented tests following the guidelines in: "The good: Test".
• Make sure you have updated the cookbook with examples and showcases (for bug fixes, enhancements & new components).

Review

• Determine if your changes are a fix, feature or breaking-change, and add the matching label to your PR. If it is tooling, dependency updates or similar, add ignore-for-release.
• Do a self-review.
• Request that the changes are code-reviewed
• Request that the changes are UX reviewed (only necessary if your PR introduces visual changes)

When the pull request has been approved it will be merged to develop by Team Kirby.

[Fuzzy3]

ListItemTemplate is also marked as standalone and should be imported directly where needed.
import { ListItemTemplateDirective } from '@kirbydesign/designsystem/list';

[Fuzzy3]

Nice ! I have one comment about adding an "X" to the template name, and then I'd like if we could update the storybook documentation, such that it is very clear that this template directive is needed.

Also the title and description should be updated in this PR to reflect the solution 🤓