NPCs keep track of total sales

[eltank]

Summary

Infrastructure "NPCs keep track of total sales"

Purpose of change

This change makes trading NPCs keep track of the total value of everything you ever sold to them.
This sales value can be used in dialogue scripts for dynamic lines and conditional dialogue. It can also be modified via dialogue.
Primarily intended for the new merchant Rubik, but can probably be used to unlock some new options with the Free Merchants as well (#51320).

Describe the solution

Add sold to npc_opinion, stored in npc::op_of_u.
Modify the "Edit NPC" debug menu to show this value and allow changing it.
Update op_of_u.sold in trading_window::update_npc_owed using trading_window::your_sale_value (also introduced in this PR; computed in trading_window::perform_trade).
Add talker::sold() and talker::add_sold() and implement them in talker_npc similar to debt() and add_debt().
Add code to handle "sold" in the "u_val" handler in condition.cpp.
And some code cleanup related to npc_opinion.

Describe alternatives you've considered

Implementing u_have_sold as a special condition similar to u_are_owed. However, that's much less flexible in terms of possible comparison operations and doesn't allow modification. [Edit: I just realized that I'm using features that were recently added in #50305. This would not have been possible more than 2 weeks ago. Thanks, @Light-Wave!]

Testing

Modified the Refugee Center merchant json:

    "responses": [
      { 
        "text": "Can I get a discount?",
        "topic": "TALK_DONE",
        "condition": { "compare_int": [ { "u_val": "sold" }, { "const": 1000 } ], "op": ">=" }
      },
      { 
        "text": "Forget I ever sold you anything!",
        "topic": "TALK_NONE",
        "effect": { "arithmetic": [ { "u_val": "sold" }, { "const": 0 } ], "op": "=" },
        "condition": { "compare_int": [ { "u_val": "sold" }, { "const": 0 } ], "op": ">" }
      },
...
  {
    "id": "TALK_EVAC_MERCHANT",
    "type": "talk_topic",
    "dynamic_line": { "compare_int": [ { "u_val": "sold" }, { "const": 1000 } ], "op": ">=", "yes": "Welcome, big spender…", "no": "Welcome…" }
  },

patch.txt

Teleported to the Refugee Center. Traded 1 Hub01 coin for 1 pipe bomb.
Debug menu for the merchant shows:

The conversation changes to:

After selecting the option to forget the sale:

Additional context

[eltank]

Tagging in @I-am-Erk

[Theawesomeboophis]

Awesome addition, although I usually just steal everything they have and kill them anyways. Great work.