Let the bulk of the push_error() calls take a Spanned trait impl, so
that we can pass node on the call sites. Then when later change the
underyling trait to pass something that can also provide the source file
and we don't have to change all call sites again.
Using the commands property we can just paste SVG paths. This makes it
much easier to write examples/demos. A good online path designer is
for example https://codepen.io/anthonydugois/pen/mewdyZ
Require that the values to the path element properties are numerical literals.
The conversion happens in the path "compilation" pass, where we can do more
pre-calcuation in the future.
