[StringBuilder] Added StringBuilder remove/discard methods.
[StringBuilder] Added StringBuilder remove/discard methods.

- StringBuilder.discard_from(index:Int32)
- StringBuilder.remove_at(index:Int32)->Character
- StringBuilder.remove_first()->Character
- StringBuilder.remove_last()->Character

AbePralle/Rogue,AbePralle/Rogue,Plasmaworks/Rogue,Plasmaworks/Rogue,Plasmaworks/Rogue,Plasmaworks/Rogue