feat(scheduler): add interval

[d3vyce]

Hi!

As discuss in #290 #348

In this PR, I'm adding the ability to run scheduled tasks every X seconds. This fills a gap that CRON doesn't allow. To avoid loop invocation spam, I calculate the next run based on which tasks are closest to executing. The next run is therefore dynamic.

This PR is still a WIP, but I welcome feedback to adjust the code if necessary. I still have to do:

• [x] Base implementation
• [x] Documentation
• [x] Clean code
• [x] Tests