[css-forms-1] define how `::picker` works with input pickers

[sorvell]

The unique picker name appears only to be defined for select, but it seems like this should include all the input types that support pickers. This is likely not exhaustive, but some examples include:

• date
• time
• datetime-local
• month
• week
• color
• file
• autocomplete ?
• search ?

Presumably styling picker content is out of scope for forms-1; however, it'd be great if the spec could call this out explicitly.