New `FieldSet` component

[amcclain]

I've reached for this a few times, and @pauldelano just asked about such a component to help wrangle increasingly complicated forms in a client project.

This would be a lightweight container for one or more FormFields (or any other components, really - other layout layers will typically be needed for internal arrangement). Main features are a thin outline styled by default + a title that's inline with that outline.

See the raw HTML control as a reference example.

As discussed with Paul, we do often use Panels with compact headers for internal form organization. Their support for additional features like headerTools can be helpful. So we should be sure a new component would add value and clearly be distinct from panel in a helpful way.