feat: support every song difficulty including Easy and Normal

src/lib/data/difficulty.ts

@@ -0,0 +1,10 @@
+export const difficulties = ['expert', 'hard', 'normal', 'easy'] as const;
+export type DifficultyKey = typeof difficulties[number];
+
+export function isDifficulty(value: unknown): value is DifficultyKey {
+  return typeof value === 'string' && difficulties.some((difficulty) => difficulty === value);
+}
+
+export function difficultyLabel(value: string): string {
+  return value.charAt(0).toUpperCase() + value.slice(1);
+}