API Review: Custom ScrollBar Style

[dianaqu]

API Review

Title: Custom ScrollBar Style

Developers would like to be able to set the ScrollBar to Windows fluent overlay style and
match the Windows theme. Some customers have used browser flags but they change names and behavior and are not reliable. Thus we want
to provide an extendable API that allows developers to set ScrollBar style. Currently,
only Windows fluent overlay are supported, adding new styles in the future will not introduce
any breaking changes.

API Spec: MicrosoftEdge/WebView2Feedback#4221

Please review the pull request and add any feedback you have about this API. We appreciate your input and support!