Support Nested Fieldsets in Forms 2.0

Straightforward request: both the W3C and WHATWG consider nested fieldsets to be valid HTML.

• https://dev.w3.org/html5/spec-LC/forms.html#the-fieldset-element
• https://html.spec.whatwg.org/multipage/form-elements.html#the-fieldset-element

I would like the ability to use nested fieldsets inside the Marketo UI for second- and third-level groupings for forms. Having this work inside the UI rather than in custom code ensures marketers can control any level of nesting needed for their forms.