`IFileType.icon` now working in rendermime-interfaces;
`IFileType.icon` now working in rendermime-interfaces;

- fully functional example of `icon: string | {name, svgstr}` interface
can be found in (the rendermime-interfaces based) vega5-extension pkg

- initial testing indicates that the rest of the icons in jlab also
made it through the recent changes mostly (completely?) unscathed

- there do seem to be some real CI failures in test-docregistry

jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab