<slot> can be nested

[Rich-Harris]

I misread this — it turns out you can nest <slot> elements in web components, so we should too:

<b>Name</b>:
<slot name="fullName">
    <slot name="firstName"></slot>
    <slot name="lastName"></slot>
</slot><br>
<b>Email</b>: <slot name="email">Unknown</slot><br>
<b>Address</b>: <slot name="address">Unknown</slot>

If no slot='fullName' element was provided, it would render the slot='firstName' and slot='lastName' ones instead. Should be straightforward, I think it's just a case of removing the validation.