decide whether to support `<style>` element; or explicitly error out

currently we only support style _attributes_ but not a `` element. When the latter is present, picosvg ignores it silently but then produces incorrect results (e.g. if the `fill` attributes are defined within the embedded stylesheet, then the paths will appear black).

I propose for now we raise an error and request the user to fix the input.

The issue names no source file or test. Start by locating the SVG handling for elements and existing error tests; done means embedded styles no longer produce silently incorrect output, with the chosen behavior covered by a test.

Written by the indexing model from the issue text.