There are two methods to get the host port from `PortProvider`:

- `#hostPortFrom` which returns the host port exclusively from the
  `sidebar-host` channel.

- `#on` allows to register a listener that is fired upon requests
  of host frames for channels other than the `sidebar-host`.

This is an attempt to simplify the API of `PortProvider` and merge both
methods into one.

Advantages:

- simplifies the `Sidebar` constructor signature (no need to pass the
  host port)

- avoids the pitfall of creating host-sidebar RPC channel before
  registering the RPC methods

- prepares for future guest-host channel see https://github.com/hypothesis/client/blob/ad93317debdac0d2c3abec83eb9d690934632ef3/src/annotator/index.js#L64-L66 (where the `#on` method is use to create the guest-host channel).