OZG-6643 OZG-6765 Add fieldset component

alfa-client/apps/demo/src/app/app.component.html

alfa-client/apps/demo/src/app/app.component.ts

alfa-client/libs/design-system/src/index.ts

...libs/design-system/src/lib/checkbox/checkbox.component.ts

...design-system/src/lib/fieldset/fieldset.component.spec.ts

...libs/design-system/src/lib/fieldset/fieldset.component.ts

...t/libs/design-system/src/lib/fieldset/fieldset.stories.ts

...lient/libs/design-system/src/lib/tailwind-preset/root.css

.../design-system/src/lib/tailwind-preset/tailwind.config.js