Setting "importsNotUsedAsValues": "error"

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

front/src/Phaser/Map/LayersIterator.ts

@@ -1,4 +1,4 @@
-import {ITiledMap, ITiledMapLayer} from "./ITiledMap";
+import type {ITiledMap, ITiledMapLayer} from "./ITiledMap";
 
 /**
  * Iterates over the layers of a map, flattening the grouped layers