feat: New mod: Skills Through Kills

[Coolthulhu]

Purpose of change (The Why)

Alternative way of gaining skills: murder. "Learn by doing" is, depending on skill, very hard to impossible to balance. Learning knitting by shanking zombies might not be realistic, but is much cleaner design-wise.
Skills Through Kills incentivizes going out and fighting, which should be the focus of the game.

Describe the solution (The How)

New mod that does the following:

• Disables normal skill gain (for technical reasons, it has to work like this at the moment)
• Conflicts with Stats From Kills (also technical reasons)
• Disables booklearning
• Adds option to increase skills when your kill_xp - total_skill_xp(right_now) > total_skill_xp(with_skill_increased_by_1)

Describe alternatives you've considered

• Implement booklearning in this PR. It would work by effectively lowering the cost of raising a skill by 25%.
• Implement NPC learning in this PR. Right now, they just don't gain any xp.
  • Could just have them keep learn-by-doing, but didn't do it here for technical reasons

Testing

• Start new character with 0 in all skills
• Try to level a skill - should fail with msg telling you how much xp you need
• Spawn some big gun and 20 hulks
• Kill the hulks with the gun
• Level a skill - it should allow it. It should actually increase the skill too.
• Level a different skill - the cost should be higher than before, but still within reach

Additional context

Checklist

• Add xp display somewhere
• Add xp cost display to skills in @
• Refine the formula of skill gain so that early game is easy, but late game is hard

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.
• I've committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.
• I understand that, unless specified otherwise, my contributions will fall under the AGPL v3.0 and/or CC-BY-SA 4.0 licenses

[scarf005]

it errors of missing "xp.h" file.