Warn users for firing commands that doesn't exist

> I think it is a good idea to show warning to any other commands other than what is available.

For instance, if the user makes a typo `mern inis` instead of `mern init` as per now there is no message being shown.
A warning might be suitable here such as `unknown command inis`

使用 `mern inis` 重現該行為，然後找出 CLI 命令解析或分派的進入點。為 `unknown command inis` 這類無法辨識的命令新增警告，並驗證 `mern init` 這類有效命令仍可正常運作。

由索引模型根據 Issue 內容生成。