Add CI test for package exports

[dstaley]

Based on the cool idea from then/is-promise#25, I think it'd be a great idea to add a new GitHub workflow check to confirm that our exports are properly importable in various environments.

I think we should probably check:

• Node (both current and supported LTS versions)
• Rollup (to confirm ES modules import correctly)
• Webpack (not totally sold on this one, but I'll think about it)