* add interface for stats, d2 is an implementation
* fix incorrect comment, remove ennecessary int
* simplified description functions, remove duplicates
* moved default stringer functions
* fixed incorrect stat combine method
* change `Create` to `New` in method names
* d2stats + diablo2stats refactored again
- simplified `NewStat` provider function
- added initializer for stat values that sets the stringer functions, value types, and combination types for values when created
- removed redundant description functions
- added stat value combination types `sum` and `static`
`static` stat values which are not altered when stats are combined. this makes sense for stats like proc-on-hit or +skills to class
example:
Stat A: `10% reanimate as: skeleton mage`
Stat B: `8% reanimate as: skeleton archer`
Stat C: `6% reanimate as: skeleton archer`
A and B can not be combined
B and C can be combined to `14% reanimate as: skeleton archer`
