core: Option to skip "unused" item links (#4608)

* core:  Option to skip "unused" item links

* Update worlds/generic/docs/advanced_settings_en.md

Co-authored-by: Exempt-Medic <60412657+Exempt-Medic@users.noreply.github.com>

* Update BaseClasses.py

Co-authored-by: Scipio Wright <scipiowright@gmail.com>

---------

Co-authored-by: Exempt-Medic <60412657+Exempt-Medic@users.noreply.github.com>
Co-authored-by: Scipio Wright <scipiowright@gmail.com>

worlds/generic/docs/advanced_settings_en.md

@@ -214,12 +214,13 @@ Timespinner:
   progression_balancing: 50
   item_links: # Share part of your item pool with other players.
     - name: TSAll
-      item_pool: 
+      item_pool:
         - Everything
       local_items:
         - Twin Pyramid Key
         - Timespinner Wheel
       replacement_item: null
+      skip_if_solo: true
 ```
 
 #### This is a fully functional yaml file that will do all the following things:
@@ -262,7 +263,7 @@ Timespinner:
   * For `Timespinner` all players in the `TSAll` item link group will share their entire item pool and the `Twin Pyramid
     Key` and `Timespinner Wheel` will be forced among the worlds of those in the group. The `null` replacement item
     will, instead of forcing a specific chosen item, allow the generator to randomly pick a filler item to replace the
-    player items.
+    player items. This item link will only be created if there are at least two players in the group.
 * `triggers` allows us to define a trigger such that if our `smallkey_shuffle` option happens to roll the `any_world`
   result it will also ensure that `bigkey_shuffle`, `map_shuffle`, and `compass_shuffle` are also forced to the
   `any_world` result. More information on triggers can be found in the