For all length specifications, use proper units of measurement

[njbart]

Bug description

For all user-facing length specifications, please refrain from supporting dimensionless numbers implied to be inches.

Do support numbers only when combined with explicit units of measurement, such as 10in, 17cm, or 150mm.

(Including, but by no means limited to page-width, fig-width, fig-height …)

Please do follow the examples of TeX/LaTeX, pandoc, and pretty much every text processor (MS Word, LibreOffice, Pages, etc.) there is in this respect.

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.

We will eventually need to bring this to inches as knitr and other libs need these, we would need to convert.

[mcanouil]

For reference:

• feat: allow specification of height and width for landscape  #12921 (comment)

@gordonwoodhull What do you think if we took the code you wrote in src/resources/filters/modules/typst_css.lua and made it a general-purpose Lua module to be used across our filters? It would require a bit of refactoring, but I think it's worth it.

• https://github.com/quarto-dev/quarto-cli/blob/main/src/resources/filters/modules/typst_css.lua