feat: add CSS/SASS variables for ul > li vertical alignment

[ghiscoding]

• provide ways to use flexbox, instead of list-items when possible for vertical alignment

[github-actions]

🎭 Playwright Summary - Success 🎉

Playwright Report

multiple-select-root@0.0.0 test:e2e:ci /home/runner/work/multiple-select-vanilla/multiple-select-vanilla
playwright test --config playwright/playwright.config.ts

Running 63 tests using 1 worker
·······························································
63 passed (1.2m)