feat(positions-enum): add a static positions property on the Popover component

[jaebradley]

Summary

When using this component, we have an object that basically serves as an enum for the various position preferences we might need for different Popover use-cases.

I was hoping of adding this enum as part of the public API for this library.

Where To Add The enum

I noticed that this library has a single module.exports value (the Popover component) which is why I went with the static property on Popover directly vs. adding a named property on the module.exports object that would reference the POSITIONS enum object (my understanding is that doing something like module.exports.POSITIONS = lib.POSITIONS would effectively be like adding the static property anyways since module.exports would reference the exported Popover component).

Testing Modifications (i.e. Storybook)

I modified the underlying Storybook stories to use the new POSITIONS enum (exported from layout.js) to sanity check my changes.

Feel free to modify the naming pattern for the enum - I've just seen a general pattern of UPCASE enum names and UPCASE for the underlying property names