CSS4 box-shadow variable for toolbar-background

# Feature Request
Please add a css4 box-shadow variable for ion-header ion-toolbar:last-child or ion-footer ion-toolbar:first-child.

**Ionic version:**
[x] **4.x**

**Describe the Feature Request**
Ionic 4 has no satisfying option to add a box-shadow to the header in iOS mode. Adding a box-shadow to the ion-toolbar or ion-header does not work, because it leaves an ugly navigation animation if the header is not the same size on the parent / child pages.


**Describe Preferred Solution**
CSS4 box-shadow variable for ion-header ion-toolbar:last-child or ion-footer ion-toolbar:first-child. This was the fix for adding a good looking box-shadow to toolbars in Ionic 3 and is prevented with the new shadow DOM.

**Additional Context**
The solution given in https://github.com/ionic-team/ionic/issues/16895 does not fix the problem because then the navigation animation looks bad.

先定位 ion-header、ion-footer 和 ion-toolbar 的 iOS-mode 樣式，然後檢視現有 toolbar CSS 變數的定義方式。針對已回報的導覽動畫情況驗證此變更，並確認要求的 box-shadow 變數可以在不改變版面配置的情況下，為相關的第一個或最後一個 toolbar 設定樣式。

由索引模型根據 Issue 內容生成。