Tweak flag classes for Library (#7829)

* Tweak flag classes for Library

- Add interface for Flag and Mask
- Merge Sort Type and Direction into one class
- Use custom serializers for preferences
  - Mainly to not break the old

* Review changes

app/src/main/java/eu/kanade/data/category/CategoryRepositoryImpl.kt

@@ -11,6 +11,10 @@ class CategoryRepositoryImpl(
     private val handler: DatabaseHandler,
 ) : CategoryRepository {
 
+    override suspend fun get(id: Long): Category? {
+        return handler.awaitOneOrNull { categoriesQueries.getCategory(id, categoryMapper) }
+    }
+
     override suspend fun getAll(): List<Category> {
         return handler.awaitList { categoriesQueries.getCategories(categoryMapper) }
     }