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

[jaebradley]

I don't know what your browser compatibility guarantees are (sorry if I missed this in the documentation somewhere) but Object.assign is not compatible with IE and it doesn't look like this gets polyfilled when building locally (at least for me).

Happy to add a polyfill for this or to build the object without using Object.assign.