Add findFirst utility method

還沒有人認領這個 Issue。

**Is your feature request related to a problem? Please describe.**
When you want to find a value just by the key, but don't care about the structure, you most likely write a recursive method to do that for you. So I think it would be good to include this utility method into the project.

**Describe the solution you'd like**
Basically a recursive method for both `AbstractObject` and `AbstractArray`.
`AbstractElement findFirst(String key)`
The method should be on AbstractArray as well, because an AbstractArray could contain objects which contain the key I want to search for.

先定位 AbstractObject 和 AbstractArray，並閱讀它們如何表示巢狀元素。在兩個進入點中實作所要求的 findFirst(String key) 行為，遞迴搜尋巢狀物件和陣列；完成的標準是兩種類型都能不論結構如何找到所要求的鍵。

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