[css-shapes] Replace `<shape-arg>` with `<length-percentage>`

[dholbert]

(Upgrading bugzilla-tracked bug https://www.w3.org/Bugs/Public/show_bug.cgi?id=29598 to a github issue):

CSS Values and Units 3 now has ``, so the `` type could be dropped from CSS Shapes Module Level 1 and replaced with ``. That would reduce the indirection through the types and make them more directly visible.

There are also multiple occurrences of "<length> | <percentage>" and the inverse "<percentage> | <length>" that should be changed to <length-percentage>.

Links:

https://drafts.csswg.org/css-values-3/#typedef-length-percentage
https://drafts.csswg.org/css-shapes-1/#typedef-shape-arg

[dholbert]

[CC @jwatt, @astearns, @vhardy, @atanassov)

[tabatkins]

Fixed.