DS-66: Borders

Add borderWidth variables and a story to display those variables. Add also a global style to set box-sizing: border-box to all elements.

Demo: https://storybook-qa.it.helsinki.fi/ds-66-borders/?path=/story/foundations-borders--borders

Jira: https://jira.it.helsinki.fi/browse/DS-66

Figma: https://www.figma.com/design/XHPvq3wuJA9eYAbU9b9NeI/University-of-Helsinki-Design-System-%E2%80%93-2024?node-id=1704-23502&node-type=canvas&t=PWu7ZNEMnWoJayjV-0