Add <style> element

[Jean28518]

Hey :) First of all, good work!

I have the issue that a specific .svg can't be rendered correctly:

══╡ EXCEPTION CAUGHT BY SVG ╞═══════════════════════════════════════════════════════════════════════
The following UnimplementedError was thrown in parseSvgElement:
The <style> element is not implemented in this library.

Style elements are not supported by this library and the requested SVG may not render as intended.
If possible, ensure the SVG uses inline styles and/or attributes (which are supported), or use a
preprocessing utility such as svgcleaner to inline the styles for you.

Picture key: /usr/share/icons/hicolor/scalable/apps/thunderbird.svg
════════════════════════════════════════════════════════════════════════════════════════════════════
flutter: unhandled element filter; Picture key: /usr/share/icons/hicolor/scalable/apps/thunderbird.svg

The rendered output:

The file to render:
thunderbird

So my proposal would be, to add the implementation for the <style> element.