feat: weight and push factor constants, more Z axis work, fixes

[potsmugen]

Feat:

• Added projectile collision check in Z axis
• Two new [Size] constants: Weight and PushFactor
• Weight constant makes heavier characters push lighter characters more. Default 100
• Push factor controls how smoothly player overlap is resolved. A lower value makes players push away from each other more slowly. Default 1.0

Fix:

• Fixed issue where fall envshake would set the wrong frequency, making the shake invisible
• Reversaldefs can now assign fall envshake gethitvars. Fixes #2012

Refactor:

• Refactored Z axis collision check so it can check either chars or projectiles
• Applied a tentative formula for drawing order when Z axis is enabled. Objects will be layered according to "sprpriority + pos z"
• Projectiles act a bit more like Mugen when they trade hits with each other, pausing in place while they do it
• Moved EnvShake code to system.go since it doesn't belong to stages where it was
• Afterimages now decrease in drawing priority over time, as in Mugen