Add port for Qt6

# Motivation

I developed an application in Qt6 that displayed SVG, but the display was grossly inaccurate.
Qt6 has its own implementation, but there is little room for improvement.

So I searched for a reusable SVG parser and found your project.
I added a port for Qt6.
No changes were made to the existing code.

## Modified

- Add svgnative/include/svgnative/ports/qt/QSVGRender.h
- Add svgnative/example/testQt

Start by reviewing svgnative/include/svgnative/ports/qt/QSVGRender.h and the svgnative/example/testQt entry point. Compare the Qt6 port with existing ports and verify that the example demonstrates SVG rendering accurately; done means both named additions are integrated without changes to existing code.

Written by the indexing model from the issue text.