Get rid of padding alignment hack once Safari supports `align-items: baseline` for blank inputs

[adamkudrna]

See https://bugs.webkit.org/show_bug.cgi?id=142968

1. Change default label and field alignment to real baseline:

   --rui-form-field-horizontal-label-vertical-alignment: baseline;
   --rui-form-field-horizontal-field-vertical-alignment: baseline;
   --rui-form-field-horizontal-label-padding-y: 0;

2. Get rid of FormLayoutCustomField's innerFieldSize prop.

Make sure various label and field alignment scenarios are still supported by us—possible scenarios may narrow down by the change. Consult with src/lib/styles/tools/form-fields/_box-field-layout.scss comments.

Current state of baseline field alignment in Safari 14.0.3:

Almost perfect (quick, no code tweaks) baseline alignment in Firefox:

Be careful when tap target is involved in calculations:

Since this works also for FormLayoutCustomFields, we could get rid of their innerFieldSize prop:

[adamkudrna]

Comes from #263.