Add placeholder support for jump mechanic

Summary

I'd like to be able to use variables and/or placeholders for the jump mechanic

Usage

You would be able to use a variable or placeholder in the place of the velocity option for jumping.

Value

In the case of my project, I'm making a skill that allows the mob to jump at a height based on how far above the target is from the mob. This way, the mob doesn't over or underjump when using it's skill.

Priority

I would like a high priority as I'm hoping to release my mob to the public soon, and I want the abilities to be perfected. I can wait though as it isn't gamebreaking when using a constant value.

Implementation

Simply be able to do something like - jump{v=}