Trailing icon now a part of utills and can use external icon

[AgreSanGit]

Which issue does this PR close?

This PR closes #3042

What is the new behavior?

Trailing icon is now a part of Kirby utils and can be used to set a custom trailing icon. Apart from Kirby icons it is now also able to use external icons. Cookbook link showcase is updated.

Does this PR introduce a breaking change?

• Yes
• No

Are there any additional context?

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.

[jakobe]

Hey @AgreSanGit ,
Implementation looks great 👌🏻
I have a few suggestions for improving and clarifying the docs 🤓