Simplifying percentage syntax in width/height attributes

[Omikhleia]

Whether in Djot or Markdown, for key-value attributes used as dimensions, we currently state "that any unit system supported by SILE is accepted"

Hence, e.g. ![](someimage.png){height="4cm"}

An implication, though, is that we need to use width="50%lw", or height="50%fh" for frame-relative sizing... This is not very user-friendly, and probably not what most users would expect.

At our level, we can't know whether those attributes will eventually end up as SILE measurements/lengths. Of course, it could be better if SILE did it natively, but that's a bunch of packages to address (such as image and svg in the core distribution, my own embedders, ptable etc. -- and probably others I overlooked here...)

A proposal would we to:

• Automatically convert "50%" to "50%lw" for width (note the "lw" rather than "fw" -- I'd tend to think this is the most correct default interpretation)
• Automatically convert "50%" to "50%fh" for height