Proposal: TabView customizable scroll amount for decrease/increase buttons

# Proposal: TabView customizable scroll amount for decrease/increase buttons

## Summary
When using large tab headers, the default scroll amount that is applied is rather small (50px). This will require a user to click the scroll increase/decrease buttons multiple times (or hold them), in order to scroll a single tab item. This could be prevented with a customizable scroll amount.

## Rationale
* Implementing this feature would increase the user-friendliness of the control
* There is already a TODO marked in the [TabView.cpp](https://github.com/microsoft/microsoft-ui-xaml/blob/main/src/controls/dev/TabView/TabView.cpp)
```csharp
// TODO: what is the right number and should this be customizable?
static constexpr double c_scrollAmount = 50.0;
```

Start with src/controls/dev/TabView/TabView.cpp and inspect the TODO around the fixed 50px c_scrollAmount value. Trace how the decrease and increase buttons use that value, then determine the appropriate customizable control surface and verify that both buttons honor it for large tab headers.

Written by the indexing model from the issue text.