Update item_groups.json

#### Summary
```SUMMARY: Content "reweight gemstones in item_groups.json"```
#### Purpose of change

Diamonds and pearls are more commonly worn gemstones in New England.
#### Describe the solution

The item group is subtype is not defined. Therefore it defaults to old, which works as a distribution.

The item_group previously had each item weighted 50. All were equally likely.

I&CleverRaven#39;ve changed diamond and pearl to 100. So now the probability is:

* diamond: 12.5%
* pearl: 12.5%
* any other: 75%
#### Describe alternatives you've considered

I considered making emeralds less common due to their rarity, but I couldn&CleverRaven#39;t determine an appropriate ratio.
#### Testing

None! I did this through github in the browser so no testing was done.
#### Additional context

I want to add turquoise to the table too, but I figured there might be a good reason why this common gemstone was left off.

data/json/itemgroups/item_groups.json

@@ -3102,7 +3102,7 @@
     "type": "item_group",
     "items": [
       [ "citrine", 50 ],
-      [ "diamond", 50 ],
+      [ "diamond", 100 ],
       [ "emerald", 50 ],
       [ "peridot", 50 ],
       [ "amethyst", 50 ],
@@ -3111,7 +3111,7 @@
       [ "blue_topaz", 50 ],
       [ "tourmaline", 50 ],
       [ "alexandrite", 50 ],
-      [ "pearl", 50 ],
+      [ "pearl", 100 ],
       [ "opal", 50 ],
       [ "ruby", 50 ],
       [ "garnet", 50 ]