Add flag to determine if wheel event delta is from direct interaction or momentum / inertia / kinetic scrolling

[flackr]

See https://github.com/vmpstr/htmldemos/blob/master/gestures/wheel-momentum.md for details. The TLDR is touchpads on all platforms have an inertial scrolling phase which in native code can be detected. Implementing this would allow authors to handle swipes or provide their own inertia handling when the application has additional information about where swipes should settle.

TLDR, can we add a way of detecting this and if so, what's the best attribute name / values?

@vmpstr

[mustaqahmed]

Wondering if this discussion should belong to the UIEvents spec instead.

Alternatively we can consider moving WheelEvent definitions from UIEvents to PointerEvents. In TPAC 2024 (see WebApps WG Day 3) we agreed on a similar move for MouseEvents but I don't recall discussing a similar move for the WheelEvent subclass!