USWDS - Core: Add transition utility with reduced motion preference handling

[aduth]

Summary

Animated transitions now respect the system's reduced motion preference. A new transition utility handles default behavior for easing and disabling inessential animation.

Breaking change

This is not a breaking change.

Related issue

Closes #5256

Preview link

In local Storybook, can be previewed at:

• http://localhost:6006/?path=/story/components-tooltip--tooltip
• http://localhost:6006/?path=/story/components-skipnav--skipnav
• http://localhost:6006/?path=/story/components-modal--default

Problem statement

As a user who may be distracted or experience harm such as nausea due to animated content, I expect that the U.S. Web Design System will respect my preference to experience reduced motion where possible.

Related:

• WCAG Success Criterion 2.3.3: Animation from Interactions (Level AAA)

Solution

Adds a new utility u-transition with smart defaults to handle:

• Default easing
• Optional $essential argument (default false) to control whether transition is disabled when reduced motion is preferred

Related:

• WCAG Technique C39: Using the CSS reduce-motion query to prevent motion

Testing and review

See "Preview links" above.

Unless system preferences are configured to prefer reduced motion, there should be no changes in the appearance of animated transition effects.

Instructions for configuring reduced motion preference:

• macOS: https://support.apple.com/guide/mac-help/stop-or-reduce-onscreen-motion-mchlc03f57a1/mac
• Other: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-reduced-motion#user_preferences

---

Before opening this PR, make sure you’ve done whichever of these applies to you:

• Confirm that this code follows the 18F Front End Coding Style Guide and Accessibility Guide.
• Run git pull origin [base branch] to pull in the most recent updates from your base and check for merge conflicts. (Often, the base branch is develop).
• Run npm run prettier:sass to format any Sass updates.
• Run npm test and confirm that all tests pass.
• Run your code through HTML_CodeSniffer and make sure it’s error free.