calc_interest, and variables to better modify interest

[bepisfever]

Adding 2 global variables that can be changed to modify interest more easily without modifying the behavior of vanilla interest-related variables.

• G.GAME.extra_interest_cap: Adds onto the current interest cap.
• G.GAME.extra_per_interest: Adds onto the amount of money per $1 interest.

SMODS.get_interest() is added, which returns the total interest cap, and total cash per interest.

And Jokers can now have calc_interest(card):

• Returning add_cap will increase interest cap by that amount.
• Returning add_per_interest will increase cash per interest by that amount.

Additional Info:

• I didn't modify api's or I've made a PR to the wiki repo.
• I didn't modify api's or I've updated lsp definitions.
• I didn't make new lovely files or all new lovely files have appropriate priority.

[english5040]

Very reasonable, making interest easy to modify is a good idea. I remember the getter contexts thing; that seems applicable to this, you may need to update this PR.

About API structure: I am concerned about Card:calculate_interest(), because you're adding on another function onto every card. It feels a bit unnatural.