Create Fieldset component

This should include <fieldset> & <legend>. The same package could also have components for input wrappers that could be used by normal inputs, selects, etc with <label>, error messages, etc so the need for duplication is less. See https://jokul.fremtind.no/komponenter/inputgroup for inspiration

Refactor form components to have more of the similar logic.

Also discuss if we should make breaking changes to the APIs of the form components: Select, Input, Radio & Checkbox to extend the input element props rather than having a separate prop for those, ie selectProps