Fix: Jump state reset and multi-jump spam in PhysicsEnginePlatformer

[ShahrukhAbrar]

Fixes # 1848

• Added jump_cooldown to PhysicsEnginePlatformer.
• Fixed grounded check logic in can_jump.

[Cleptomania]

I wonder if we should consider adding delta_time as a parameter to the update method on Physics Engines such that we can use that for time tracking as opposed to time.time(). The built-in time function is known to have precision problems and can't really be relied on in increments less than 1 second.

That's gonna incur a lot of update work on documentation/example code though, so that's perhaps out of band of this PR. We could default it to 1 / 60 to keep backwards compat, but given we're working on 4.0 versions right now, I think it's best to break the API and assume that when using the built-in physics engines you need to passthrough the delta_time, at least for something like the Platformer one and any equally or more advanced than that.

[ShahrukhAbrar]

Yeah that makes sense, Since I am already in this part of the code, would you like me to refactor the update method to accept delta_time and update the jump tracking logic to use it, or would you like to handle this in a separate PR?