feat: make gravity scale a vector to change gravity direction at runtime

[amyspark-ng]

Would it be possible to implement a system where the gravity for body() components changes direction at runtime? (replit/kaboom#575)

Was in the original kaboom issues tab, thought it was still relevant, so it might be a nice feature

Originally posted by @amyspark-ng in #39

[amyspark-ng]

isGrounded() won't work if the gravity is negative #689
replit/kaboom#689

i don't know if this was fixed but might still be relevant

Also got me thinking on maybe creating a different function for body component called addForce, where it's like jump, but you can also set horizontal force applied?

[mflerackers]

I'll add it once I refactor gravity. I'll add applyForce and applyImpulse.

[mflerackers]

Right now there are tests like

isFalling(): boolean {
  return this.vel.y > 0
},

isJumping(): boolean {
  return this.vel.y < 0
},

These need to become

isFalling(): boolean {
  return this.vel.dot(gravity) > 0 // Same direction as gravity
},

isJumping(): boolean {
  return this.vel.dot(gravity) < 0 // Opposite direction as gravity
},