Add option to integrate with Foundry World Time

[DC23]

This will allow unification with game systems and other modules that also use World Time.

It should be a module setting that can be enabled, with the default to keep this module separate.

[DC23]

Some technical effort will be required. There are different hooks to respond to for time change events, Foundry world time stores time in seconds, not time in minutes, and so on. I'll need to think about how to refactor towards this change with the minimal chance of bugs and disruption to the current code.

[DC23]

Simplest initial refactoring is to change my internal time calculation to store time in seconds since the epoch rather than minutes since the epoch. Having the same internal units should make future compatibility much simpler.