Altair-viewer

- add bundled Vega v5.21.0
- add bundled Vega-Lite v4.17.0
- add bundled Vega-Embed v6.20.0

- Add bundled Vega v5.10.1
- Add bundled Vega-Lite v4.8.1
- Add bundled Vega-Embed v6.5.2

- Update bundled Vega to v5.9.2
- Update bundled Vega-embed to v6.2.2

- Update bundled Vega to v5.9.1 (23)
- Add websocket connection monitor to avoid opening multiple windows and to terminate
processes more cleanly (22)
- Fix project metadata (18, 19)

Bug: fix URLs for inline mode in JupyterLab and other non-requirejs-based renderers (19)