da-DK license plates

[woller]

Motivation / Background

This Pull Request has been created because it was not possible to generate valid Danish license plates. It adds the simplest possible license plates consisting of 2 letters and 5 digits. It follows the rules laid out on the Danish Wikipedia.

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• [x] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• [x] Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• [x] Tests are added or updated if you fix a bug, refactor something, or add a feature.
• [x] Tests and Rubocop are passing before submitting your proposed changes.

If you're proposing a new generator or locale:

• [ ] Double-check the existing generators documentation to make sure the new generator you want to add doesn't already exist.
• [ ] You've reviewed and followed the Contributing guidelines.