checkbox field misses append and prepend

As per https://learn.getgrav.org/17/admin-panel/extending#adding-a-custom-field, using `prepend` or `append` block.
But the checkbox field (and maybe others) overrides the whole `input` block **without** providing the same extension sub-blocks (namely `prepend` or `append`)

A somehow related problem is that create such an override (at `

Start with the checkbox template at , the related forms/field.html.twig guidance, and the documented prepend/append example. Compare the checkbox input block with the extension points described there; done means those blocks work for checkbox fields and a field-specific extension no longer causes the reported recursive failure.

Written by the indexing model from the issue text.