add FlxTypedWeapon

Summary of this PR:

- Added FlxTypedWeapon due to the depreciation of `setAll()`. The replacement `ForEach` requires strictly typed members.
- Moved firing logics from FlxBullet to FlxTypedWeapon.
- Enhanced the bullets creation process. Weapon now calls a custom factory function to create a bullet. (`BulletFactory` replaces `BulletType` in constructor)
- Removed `onFireCallback` and `onFireSound` (they are basically duplicates of `onPostFireCallback` and `onPostFireSound`)

Not really a fan of the `Dynamic` here... I guess you didn't want to add a type param to `FlxBullet`?

That said, why do bullets need a reference to their `weapon` at all?