Add URL Protocol Handler Registration for PWAs

[Schweinepriester]

Probably a bit early, but could become powerful:

• https://twitter.com/ChromiumDev/status/1392019338992013315
• https://web.dev/url-protocol-handler/
• https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/URLProtocolHandler/explainer.md
• Adding protocol handlers w3c/manifest#972

Might become part of https://caniuse.com/registerprotocolhandler or another entry for https://caniuse.com/?search=manifest…