This implements step 2 of the web-facing API's.
Now, the embedder receives a notification whether
a webpage wants to register/unregister a protocol
handler. They can't do anything with it yet, that's for a follow-up PR.
But locally testing shows the
following debug log:
```
pid:35180 Requesting to Register for scheme web+test with url https://web-platform.test:8443/html/webappapis/system-state-and-capabilities/the-navigator-object/%s
```
Part of #40615
Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>