A great deal of the complexity of the current code is that we must chain
2 reactive values (one in the map "GameScene.MapPlayersByKey" and one in
the snapshot store).
The new generic MapStore class can be used to listen to stores inside a map.
When the store inside the map, or the map itself is modified, the
resulting store is updated.
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
