added LootableRandom tag

[shadoxxhd]

Adds LootableRandom tag, allowing drops to be randomized:

"LootableRandom": {
      "Options": [
        {
          "ItemID": "Gear_HeatSink_Single",
          "Weight": 1.0
        },
        {
          "ItemID": "Gear_Cooling_Pod_x1",
          "Weight": 0.05
        }
      ]
    }

If "no_salvage" is set, it chooses a random option to drop instead of the original item. The normal Lootable tag is ignored when LootableRandom is set and valid, otherwise it falls back to Lootable.

Weighting is linear (drop chance is proportional to weight - P(option) = weight(option)/sum(weights)).

Since one significant use case for LootableRandom is to add rare drops to items while still usually dropping the item itself, the "no_salvage" flag is ignored for the replacement item (both for Lootable and LootableRandom).
This behavior could technically be restricted to LootableRandom, keeping the "no_salvage" behavior of the normal Lootable tag, if desired - the question is whether to simplify the behavior, or keep backwards compatibility of rare "noop" edge cases.