Standardises item blending

[SECBATON-GRIFFON]

[bugfix][consistency]

What this does

takes the blend_items lists of mortars and grinders and combines their values onto vars on types of /obj/item, any values that are higher in the grinder list were kept over the mortar's. does the same with juice_items. cuts down code to make it reused by both items. makes grinding nettles actually give their chems right for both. Closes #34237. Closes #22024.

Why it's good

consistency

How it was tested

blend_juice unit test was written for most of this, grinder and mortar in-game otherwise

Changelog

:cl:

• bugfix: Nettles and death nettles now properly grind their reagents out.
• tweak: Mortars can now grind anything an all-in-one grinder can, to the same amounts, and vice versa.
• tweak: Ground novaflowers, nettles and death nettles now grind their reagents in scale with potency.