The types for `Underline.Panel` appear to not include generic HTMLElement attributes

[joshblack]

When using UnderlinePanels.Panel, it doesn't seem to be providing type information for generic HTMLElement attributes for props that can be passed to it (like className). For example:

<UnderlinePanels>
  <UnderlinePanels.Tab>Overview</UnderlinePanels.Tab>
  <UnderlinePanels.Panel className="test">Overview panel</UnderlinePanels.Panel>
</UnderlinePanels>

In this snippet, className is not providing any type information from the underlying attribute on HTMLElement. We should update the prop information to correctly provide context into any props that it forwards along.