feat(luxon): Use DateTime.isDateTime instead of instanceof

[alex-kuck]

The usage of DateTime.isDateTime should be more stable without breaking existing behavior. It also satisfies the current version constraints of luxon (added in 1.6.0).

[alex-kuck]

For context: the current implementation using instanceof breaks, when two different versions of luxon are in use (accidentally discovered due to a misconfiguration of our bundler 😅 )