Fix alignment of small variants of radios and checkboxes

[owenatgov]

What

Re-work the small variants or the radio and checkbox components to be better aligned with their options and not use "magic numbers" to define alignment.

Why

During testing of the govuk-frontend components against the new typography scale, we noticed that both radio and checkbox small variants are now much more visibly misaligned from their labels. Because these variants are using "magic numbers" aka specific, non-dynamic numbers to define alignment, we think there's an opportunity to future proof this implementation, therefore fixing the issue found in the typography manual testing.

Who needs to work on this

Developers

Who needs to review this

Developers, designers

[owenatgov]

Giving devs until Friday morning (11th Aug) to review the spike in its present state then will start writing production code.

[owenatgov]

Moving this into blocked until 5.0 is out. At which point we'll bundle this with a few changes around refactoring to anticipate the new type scale.