[core, Lua] Implement Tredecim Scythe

[zach2good]

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What does this pull request do?

• Adds script for tredecim_scythe, with new MELEE_SWING listener
• Adds MELEE_SWING listener trigger
• Adds CLuaAttack wrapper and bindings for getting/setting critical flag of CAttack

Steps to test these changes

Use it

NOTE: The swing count is 0-indexed, so it'll crit on swing 12, and then wrap around

Closes #4819

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[zach2good]

I reckon as part of this we can/should rename the other triggers inside OnAttack, so we can better differentiate between an attack round, an attack swing, things that hit, etc.

[zach2good]

The new listener should probably be MELEE_SWING_HIT, or somesuch

[zach2good]

RIP DarkstarProject/darkstar#6424, at last