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/Items/ActionableItem.ts

@@ -4,7 +4,7 @@
  */
 import Sprite = Phaser.GameObjects.Sprite;
 import {OutlinePipeline} from "../Shaders/OutlinePipeline";
-import {GameScene} from "../Game/GameScene";
+import type {GameScene} from "../Game/GameScene";
 
 type EventCallback = (state: unknown, parameters: unknown) => void;