For PortFinder, I followed this advice:
https://github.com/hypothesis/client/pull/3881#discussion_r743030606,
except that I made the argument of `PorFinder#discover` and string
(instead of an object).

```
const portFinder = new PortFinder({ source: 'guest', hostFrame });
portFinder.discover('sidebar');
portFinder.discover('host');
```

For PortProvider, I followed this advice:
https://github.com/hypothesis/client/pull/3881#discussion_r743033013,
except that I used a getter.

```
cont portProvider = new PortProvider(...)
const bridge = new Bridge();
bridge.createChannel(portProvider.sidebarPort);
```

I have renamed the properties of `Message`:
- `source` becomes `authority`
- `channel` and `port` have been replaced by `frame1` and `frame2`

I did that to align `port1` and `frame1` and `port2` and `frame2`, while
also avoiding clashing with other names (`source`, `target`) which have
other meaning in the `window.postMessage` context.

I have removed one level of nesting in the `PortProvider#discover` that
make the method more readable.

I added some other suggestions from PR #3881.