Level should be size-aware

See the code near this line: https://github.com/ForNeVeR/O21/blob/a6b9cc232ede2b030410ee7a6fdc2545a33f73de/O21.Game/Engine/Enemies.fs#L157-L158

Currently, our `Level` is not size-aware: it's just a jagged array of cells. For some purposes (such as the linked one), we need to know the level size. We can either hardcode it (use same size from `GameRules` for every level), or calculate it in-place, but both approaches are suboptimal.

If in the future we'll want to have levels of different sizes (say for boss battles, custom mods or whatnot), hardcode will stop fitting us.

So, let's make the `Level` type to be aware of its own size.