Turning the "importsNotUsedAsValues" TS config value to "error".
This will require us to use `import type` instead of `import` when we are importing a value that is only used as a type (and therefore that is dropped by the Typescript compiler).
Why this change?
This is a requirement to be able to use Svelte in the future. See https://github.com/sveltejs/svelte-preprocess/issues/206#issuecomment-663193798
This class iterates recursively over layers, flattening groups.
This enables us to simplify the code when we iterate layers. We can remove all recursive function calls in the GameScene code (it is delegated to the LayersIterator)
