[@types/luxon] Add the invalidExplanation getters.
[@types/luxon] Add the invalidExplanation getters.

These were added as in [v1.5.0](https://github.com/moment/luxon/blob/master/changelog.md#150).
With this [PR](https://github.com/moment/luxon/pull/360).
All of the `invalidReason` and `invalidExplanation` getters can return `null` if the object is valid.

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