Further customisation of cross-ref labels

[bbartholdy]

This is related to discussion #3209 and suggested as an enhancement.

For all outputs it would be great if the (supplemental) table and figure numbers could be prefixed with a letter, e.g., naming supplementary figures and tables as 'Figure S1' and 'Table S1'. It's currently possible to use fig-title: Figure S to achieve 'Figure S 1', but it's not really ideal.

[cscheid]

This was fixed in v1.4: https://quarto.org/docs/authoring/cross-references-custom.html#example-supplemental-figures