Implement Adder Leader

[Winsalot]

Description

This PR aims to implement the Adder.

Abilities

• CausticScales: Applies damage over time and attack speed reduction for any enemy that attacks/damages this leader.

• VenomBite: Every attack applies damage over time and slow. Stacks up to 5 instances.

[AnneKitsune]

There are conflicting files

[owen-mccormick]

Some new leader-related things were just added on master. After resolving merge conflicts, it may be best to add your leader to the match statement on line 72 of spawn_leader.rs so it compiles. (This is for adding specific sprites and AI systems to leaders.) Also, a variable called leaders_vec was just added on line 480 of main.rs that leaders must be part of in order to be randomly assigned to teams.