Suggestion: mention string interning

還沒有人認領這個 Issue。

If we want to be completely clear in the [object copy lesson](https://javascript.info/object-copy), we can include a snippet that mentions string interning, where since strings are immutable, only one instance is stored in memory and any future references to that same string can be referred to the singleton in existence. So yes, primitives are "copied as a whole value" in behavior, but it's a little bit more complicated than that if we get into implementation detail.

Is this too much? Thoughts? JavaScript.info seems to be a pretty thorough resource, so I can definitely see it including this kind of detail.

從連結的物件複製課程開始，檢視課程目前如何說明基本值和字串行為。判斷字串駐留是否應納入課程，然後只有在公開討論達成決定後才更新說明；完成的標準是課程準確反映約定的實作細節程度。

由索引模型根據 Issue 內容生成。