Refactoring of updateEffects in skill.js

I did a whole bunch of refactoring to the ``updateEffects`` function of skill.js. I introduced a lot of additional functions to make the main method easier to read.
The important changes:
* The merging of the ``baseEffects`` and ``activeSteps`` is now done in an extra function ``mergeEffectsAndSteps``. I changed the main "algorithm" behind it. Like before, all attribute values are combined into a list for each attribute.
To ensure that the lists have the correct ordering, they get sorted afterwards, depending if an element is a divisor or not. This sorting can easily be extended in such a way that adding and multiplying (+ and +%) has the correct order as well - if necessary.
* The merging of the baseEffect and the upgrades into one attribute is now done in ``applyUpgrades`` (for all of them), ``applyUpgrade`` (for one specific attribute) and ``applyOperation`` (executing the number operation, e.g. x += y, x *= 1 + y, etc.).

This refactoring is the first step to enable multi staged upgrades natively (see branch [damage-stages](https://github.com/r0estir0bbe/nautsbuilder/tree/damage-stages)).