2023-01-07 Dedicated repository for on-modify hook script

[lauft]

The on-modify.timewarrior hook script now has a dedicated repository for its development.

Apart from unit tests, the repository has an end-to-end test suite, ensuring the hook script works with the combinations of both, Taskwarrior's and Timewarrior's, latest release (branch stable) and latest development (branch develop).

Changes to the script have to be made in this repository now, all present PRs will be ported to there.
Timewarrior will incorporate the current version of the script in the respective release.

The repository is currently located here, but will be integrated into the Gothenburg Bit Factory organization later.
Update 2023-03-05: The dedicated repository has been moved to the Gothenburg Bit Factory organization.