Either make QtSvg an optional dependency, or add it to the pyproject.toml extras

Pyface with the PySide6 backend currently _requires_ QtSvg: it's imported from unconditionally in `pyface.ui.qt.resource_manager`. However, recent versions of PySide6 on EDM don't include QtSvg as a dependency, leading to failures.

We should either list QtSvg as an explicit dependency in the optional-dependencies part of `pyproject.toml`, or update the `resource_manager` code to make `QtSvg` optional (it would be easy to make the import local to the place it's needed).