decrease main arena length

[TheAfroOfDoom]

the arena as it is now is suuuper long (X-axis). this was done in an attempt to be "accurate" to how long it looks in Undertale, but i think when i made it i was actually using the full screen length instead of the length between Flowey's arms

• green is what the arena's length should actually be (bounded by Flowey's arms)
• red is what it currently is meant to approximate

this change would lead to us needing to change most attack's parameters so they respect the new arena bounds. for example:

• dentata snake wall positions for bouncing
• finger guns summon positions (+ despawn timing)

the attack parameters could definitely be changed iteratively in later PRs though; we don't have to do everything all at once (yay source control + iterative development!)