Create interaction for threshold + velocity

[appsforartists]

The most useful abstraction missing from MM right now is something that synthesizes velocity and threshold. Our inputs look something like this:

• An axis
• A series of stops, where the dragged item will potentially land when released
• Some way of mapping velocity to a distance or number of stops to advance
• A velocity stream
• A translate stream

We need at least these outputs:

• preview index (e.g. which index is active, based on the current translate)
• index (only updated on release)
• destination (e.g. thresholds[index])

This has all been done in prototype code and not landed in the library because it wasn't clear what the right abstraction is. Given its a common need with complex inputs and outputs, this should probably be an interaction.

Open questions:

• What should be configurable?
  • Should the velocity mapping and drag thresholds be tunable, or shared across all uses of this interaction?
• Do we need to support both absolute and relative thresholds?
  • What about an infinitely long carousel?
  • What about a bottom sheet?
• Should it set the spring's destination upon release?
• How does it interact with application state?
• Does the application listen to this interaction's outputs and update its own state?
• Is there an index$ property on this interaction, which holds the canonical state?