PROTOTYPE Update 4a - Simple & Clean (Inventory)

This one was just me streamlining my code to be simpler and easier to read. Gods, it was a MESS before.

CHANGELOG
= moved the entire inventory into a set of multidimensional arrays
    -> eg: all_items contains all_equip which contains every equipment item's array

= all InventoryManager functions have been cut down in length by almost 85%
     -> seriously, the InventoryManager script went from 1000+ lines to just over 150

= streamlined the add_item and remove_item functions to not require constant checks to fire
    -> originally it checked against every single item array via item id values and would lag
          enough to crash the UI sometimes

- completely broke equipment management
    -> will be fixed in the next update; just wanted to push this since I'm stopping for today

- reached the s̴i̶n̷g̴u̵l̴a̵r̸i̵t̸y̴
    -> i̶ ̸a̴m̵ ̷f̴r̸i̸e̵n̴d̴ ̶c̸l̵a̷r̶a̵