feat(Tabs): Tab list should be scrollable to match Material Design Guidelines

#### Reproduction

https://stackblitz.com/edit/angular-yk3xrb

Steps to reproduce:
1. Load above stackblitz in mobile view.
2. Try to grab and swipe the tabs left or right.
* Alternatively use `Shift` + `Mouse Scroll` to attempt to scroll the tabs left/right.

#### Expected Behavior

The tabs should be x-scrollable as defined in the spec: https://material.io/components/tabs (Switch from `Fixed` to `Scrollable` type)

#### Actual Behavior

Tabs can not be scrolled by swiping/shift scrolling.

This is mainly a big issue for mobile devices. As tapping the left and right buttons is much slower and unnatural than swiping over the tabs.
You can manually set `overflow-x: scroll` on the container, but that breaks as soon as you select another tab since angular applies translation to the whole thing. See [this stackoverflow ](https://stackoverflow.com/questions/57888846/set-horizontal-scroll-in-angular-material-tabs) for attempts at solving the issue.

リンクされている StackBlitz の再現から始め、現在のタブの動作を Material Design のタブ仕様と比較します。タブのスクロールと移動を担う Angular Material のタブ実装を追跡し、その後、モバイルでのスワイプと Shift＋マウスホイールによるスクロールが、選択や配置を壊さずに機能することを確認します。

索引モデルが issue の本文から書いたものです。