introduce new player compare predicate

[Polymeta]

Hey there, first off. love the plugin, it's quite dynamic and powerful!

In the spirit of making it even better, I have came up with a new type of predicate that allows admins to specify a certain predicate that gets ran against source and target, and if they both return the same thing, the target is allowed to receive the message, this will allow for even more dynamic setups, like dynamic guild chats (or world chat written a different way). Below is an example:

  "channels": [
    {
      "name": "world",
      "enabled": true,
      "isStaff": false,
      "actionbar": "<dark_aqua>Chat Mode: <green>%world:name%",
      "prefix": "<dark_gray>[<aqua>%world:name%</aqua>]</dark_gray> ",
      "canSee": {
        "type": "advancedchat:compare",
        "compare_predicate": {
          "type": "placeholder",
          "placeholder": "world:name",
          "raw": false
        }
      }
    }
  ]

The compare predicate can be whatever predicate there is, whether builtin or mod provided, making this expression quite flexible. You could also use this to compare LP meta between users.

I tried this locally with 2 clients and it worked quite well.

[Wesley1808]

Looks good. Thanks for the PR!