Add port for Qt6

[kanryu]

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