Update to support the latest specs of scroll-driven animation APIs

[AndrewKGuan]

Creating this ticket to track the update of this polyfill to work with the latest SDA specs. Please update when status changes.

[bramus]

It’s not a full update, but this PR moves away from delay and endDelay towards using rangeStart and rangeEnd:

• #121

[bramus]

Also filed these, which are part of a spec-sync update:

• #123
• #124

[bramus]

Also related:

• #125

That should get the syntax this polyfill supports roughly in sync with the spec. Note that some of the finer details – such as the two listed above – are still missing in this polyfill.