Create exportable constants in the theme-data package

These should replace things like **constants.js** in the implementation of `Spacer` for #967.

When trying to implement a `Spacer` component that imports a list of available spacings (`['xxs', 'xs', ... 'xxl']`) from **@hig/theme-data**, the dependency caused the following crash for `Spacer`'s Enzyme tests:

Investigate and address so that `Spacer` and other packages can import from a global constants file in **@hig/theme-data**

Start by reading the existing constants.js used by Spacer and the @hig/theme-data package, then reproduce the crash in Spacer's Enzyme tests. The work is done when Spacer and other packages can import the shared constants from @hig/theme-data without causing the test failure.

Written by the indexing model from the issue text.