Flag: export FlagThemeColor type

[chi-jyskebank-dk]

Which issue does this PR close?

This PR closes # (insert issue number here)

What is the new behavior?

FlagThemColor is exported as a new type

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Are there any additional context?

This new type has been introduced in order to make it easier to pass valid colours to a Kirby Flag element.

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.