fix(header-action): allow vertical scroll when expanded

This PR hotfixes a usability issue: if the HeaderAction has vertical overflow, the overflow: hidden rule is not overridden.

This should ideally be fixed upstream:

Bug report: https://github.com/carbon-design-system/carbon/issues/17159
PR: https://github.com/carbon-design-system/carbon/pull/17160

In the meantime, however, the fix for Carbon Svelte ensures that the content is vertically scrollable (if necessary) to avoid requiring user intervention. This change can be reverted if and when this style is fixed in the upstream carbon-components library.

Otherwise, the consumer would need the following CSS:

:global(.bx--header-panel--expanded) {
  overflow-y: auto;
}

carbon-design-system / carbon-components-svelte

fix(header-action): allow vertical scroll when expanded #1992