decoration: Allow specifying amplitudes as lengths

[Jollywatt]

This allows the amplitude parameter in path decorations to be given as a length, for example:

fn(line((0,0), (4,0)), amplitude: 0.25)
fn(line((0,1), (4,1)), amplitude: 2.5mm)
fn(line((0,2), (4,2)), amplitude: t => 1em*calc.sin(float(t)*calc.pi))
fn(line((0,3), (4,3)), amplitude: (5mm, 0, 2mm, 0))

for a path decoration function fn.

[Jollywatt]

This is a follow up from #756.

[johannes-wolf]

LGTM.