refactor(real-time): move update logic to metrics package

[nicojs]

Make mutation-testing-metrics responsible for the real-time calculations in the report. This has a couple of advantages:

• Easier to reason about: the metrics are always up-to-date.
• Easier to maintain: no ad-hoc calculations are needed anymore
• Can be reused from StrykerJS itself, for example, in the console reporter

This change was already discussed here: https://github.com/stryker-mutator/mutation-testing-elements/pull/2453#discussion_r1178769871.

Note: this refactor is needed to enable fine-grained reactivity.

[nicojs]

Yeah... still needs some more work