fix: return passed value when setting db row value (#16)

Closes #14

From the example in the issue, this can now be fixed like this:
```php
final public DateTimeImmutable $date_created {
			get  => new DateTimeImmutable($this->get(Fields::DATE_CREATED->value));
			set (DateTimeImmutable $date_created) => new DateTimeImmutable(
			  $this->set(Fields::DATE_CREATED->value,
			  $date_created->format(Database::DATETIME_FORMAT)
			));
}
```

Reviewed-on: https://codeberg.org/vlw/scaffold/pulls/16

src/Database/Model.php

@@ -97,14 +97,16 @@
 		 *
 		 * @param string $key Target column to update
 		 * @param mixed $value New value of target column
-		 * @return bool Update was successful
+		 * @return mixed The value that was sent to $value
 		 */
-		public function set(string $key, mixed $value): bool {
+		public function set(string $key, mixed $value): mixed {
 			$this->_row[$key] = $value;
 
-			return $this->db
+			$this->db
 				->from($this->table)
 				->where($this->where)
 				->update([$key => $value]);
+
+			return $value;
 		}
 	}