Add ability to set SelectorButton as prefix instead of prefixIcon

[neutronstein]

Setting setSelectorButtonAsPrefixIcon to true while using OutlineInputBorder causes floating label to align to input content start. That is the default behaviour but that doesn't fit quite well in my use case. Setting the SelectButton widget as prefix suits better.

Kindly provide ability to set SelectorButton as prefix instead of prefixIcon by either adding setSelectorButtonAsPrefix field to SelectorConfig or changing setSelectorButtonAsPrefixIcon field to an enum (maybe named selectButtonPosition) with values prefix and prefixIcon.

Thanks

[Chris1234567899]

However the solution might look like, I agree on the problem. In case setSelectorButtonAsPrefixIcon is active, there should be a way to align the label to the start of the entire formfield, not the input.