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/Player/Player.ts

@@ -1,5 +1,5 @@
 import {PlayerAnimationDirections} from "./Animation";
-import {GameScene} from "../Game/GameScene";
+import type {GameScene} from "../Game/GameScene";
 import {UserInputEvent, UserInputManager} from "../UserInput/UserInputManager";
 import {Character} from "../Entity/Character";