[DONE][Suggestion] More script language features (SQRT)

[Osobist]

Hi! I have another humble request for scripts, in fact, two requests.

1) Sqrt function with lower round-off, very convenient for range calculations.

2) Dynamic change for skillIconSprite depending on tags tied with BattleUnit or skillUseUnit. With this we'll make fancy aiming mode indicator (with highlighted bodyparts).

[Yankes]

`sqrt` should be available in OXCE 7.9.17

https://github.com/MeridianOXC/OpenXcom/commit/63df39ee05c7ce984cb459a29e8be2f8df3c4064