use fraction units for typst column layouts

[gordonwoodhull]

fixes #11578
replaces #11579

Use fraction units for any defaulted or unitless column layout widths, because percent unit seems to use the percentage of the page.

I know we usually try to keep format-specific stuff out of the early filters, but the truth is that width units are not format-independent: we just assume that any widths specified are valid.

And we need to use fr and not % units for defaulted or unitless column widths for Typst.

::: {layout="[1, 2, 2, 1, 3, 1, 1]"}

::: {layout-ncol=5}

::: {layout-ncol=5}

and third image has {width=100pt}

cc @mcanouil

[cscheid]

I know we usually try to keep format-specific stuff out of the early filters

That's true and your reasoning would be good enough to justify the change. But layout filters are called in post-quarto exactly because of what you're saying: layouts cannot really be format-agnostic.

[cscheid]

Looks great! I think all we're missing here is a changelog entry.