Summary

As we've established in #7486, color computations are expensive to recalculate repeatedly and should be memoized where possible. This is fine for component styles (#7529), but our various euiComponentVariables utilities are not getting memoized except as part of component styles.

euiFormVariables is particularly chonky and likely bears examining more closely, but in the interim I'm solving this particular microperf irritant by creating a separate utility just to grab the maxWidth size for components that only need this token (a surprisingly high number).

My upcoming EuiComboBox conversion will be utilizing this PR as well for cleaner and more performant code ✨

QA

• https://eui.elastic.co/pr_7948/#/display/list-group renders at the same 400px max-width as before/as prod
• https://eui.elastic.co/pr_7948/#/layout/popover#popover-attached-to-input-element has the same 400px max-width

General checklist

• Browser QA - N/A, fairly straightforward PR that doesn't need excessive cross-browser testing
• Docs site QA - N/A
• Code quality checklist - N/A, styles only
• Release checklist - N/A, skipping a changelog as this is mostly tech debt/ an internal concern only
• Designer checklist - N/A