LG-3757: Create bundler-ready module entrypoint

[aduth]

Why: As a developer using the login.gov Design System, I expect that I can use identity-style-guide as a proper NPM package, so that I can deduplicate shared dependencies (see LG-3716) and take advantage of dead code elimination to reduce bundle size by omitting unused components.

Changelog:

• The published NPM package can now be imported into a project. Bundlers that support dead-code elimination (Webpack, etc.) can automatically remove unused code to optimize bundle size. See Usage guide for more information. Existing usage of JavaScript code from assets/ is unaffected by these changes.

Usage Documentation: https://federalist-2f194a10-945e-4413-be01-46ca6dae5358.app.cloud.gov/preview/18f/identity-style-guide/aduth-lg-3716-package-entry/usage/#use-as-a-javascript-package

History:

#148 added a main to the package's package.json, which at least allows for a developer to import the package. However, because this points to a precompiled distributable asset, dependencies cannot be deduplicated.

Impact:

Preliminary testing in identity-idp reveals bundle size reduction of 88% gzipped (451kb to 56.3kb), largely attributed to elimination of unused zxcvbn dependency.

Before	After
js/application-117899d0f4756bf36d57.js 1 MiB 2, 3 [emitted] [immutable] [big] application js/application-117899d0f4756bf36d57.js.br 402 KiB [emitted] [big] js/application-117899d0f4756bf36d57.js.gz 451 KiB [emitted] [big]	js/application-646a51c2eba309441c63.js 203 KiB 2, 3 [emitted] [immutable] application js/application-646a51c2eba309441c63.js.br 47.7 KiB [emitted] js/application-646a51c2eba309441c63.js.gz 56.3 KiB [emitted]

[aduth]

The Snyk advisory is arguably not worth concerning over, but it can't be resolved without upgrading USWDS, incidentally resolved by #159.

[zachmargolis]

LGTM, one question

[aduth]

Also, should have mentioned earlier that the "visual-regression" failure is expected: It's catching the new content added to the "Usage" page. 😅

At some point I'd like to revisit this to ideally report as a comment and not as a build failure.

[aduth]

Rebased after merging #159:

• Snyk advisory should hopefully clear up.
• Added components introduced in recent versions of USWDS (db52eee).
• For CHANGELOG.md, reconciled as "New Features" of 3.0.0 major version.

[aduth]

Visual regression shows small adjustment of padding in the header. It looks that this may have been some flakiness in the rebuild from #159, since it looks more correct in the "changes", and the "changes" are reflected even in what I see in the master branch run locally.