feat: leverage "exports" field in package.json

[Westbrook]

Description

CDNs like https://jspm.org/ generally rely on statistical analysis to decide what parts of a package to deploy to their servers. This works best when there is a large number (which we don't have) of uniform usages, but even with this, things like sp-*.js being very similar to sp-* across various build settings can lead to only some of the artifacts of a package being served. This PR expands our package.json files to support the exports field so that we can tell these sorts of tools what files we export explicitly so as to overcome our early low usage rate and any compatibility issues that might arise later of users leveraging file extension or not.

Motivation and Context

Easier end-user implementation.

How Has This Been Tested?

Previously:
https://jspm.dev/npm:@spectrum-web-components/icon@0.4/lib/spectrum-icon-checkmark-medium.css.js

Now (canary release):
https://jspm.dev/npm:@spectrum-web-components/icon@0.4.9-alpha.38/src/spectrum-icon-checkmark-medium.css.js

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[andrewhatran]

Looks good to me! The usage-based constraint of jspm is very interesting 🤔