Convert JavaScript to native ES Modules

[JohnAlbin]

We are currently distributing this packages as CommonJS files (compiled by Babel). Since we dropped support for Node.js v12 in #20, Node.js v14 and later support native ES Modules and we don't need to distribute CommonJS or use Babel.

• Update package.json to add a "type": "module".
• Convert import statements from file-extension-less references (supported by Node.js' CommonJS conventions) to fully-specified filename references supported by ES Modules.
• Remove Babel.
• Update our main entry to point at lib/index.js.
• Add exports in our package.json to remove the lib part of the path.

Follow-ups:

• 6f7b72d
• Enable Webpack 4 compatibility #39

[JohnAlbin]

Copying comment from another issue:

One reason to reconsider is that this change breaks Storybook support.

(Just discovered this when I tried to upgrade from 1.0.0-alpha.6 to your new beta.)

(I should add that I'm using Node 16 and the latest version of Storybook with its Webpack 5 option enabled.)

The commenter doesn't say which beta they were using. Beta 1 and beta 2 definitely didn't work with Webpack 4, but beta 3 does.

Re-opening until we confirm this works with Webpack 5.

[JohnAlbin]

Fixing with #46