All tests green

embassy_sync: `Sink` adapter for `pubsub::Pub`

Corresponding to the `Stream` impl for `pubsub::Sub`. Notable
difference is that we need a separate adapter type to store
the pending item, i.e. we can't `impl Sink for Pub` directly.
Instead a method `Pub::sink(&self)` is exposed, which
constructs a `PubSink`.

https://github.com/embassy-rs/embassy/pull/2969 appears to have broken
direct `publish_immediate()` on `pubsub::Channel`, as it functionally
made `PubSubBehavior` private and didn't delegate this method to the new
(private) `SealedPubSubBehavior`.

This change moves `publish_immediate`, `capacity`, and `is_full` from
`SealedPubSubBehavior` to `PubSubBehavior` in order to restore them to
`pub` visibility.

None of them are `non-exaustative`, they are all small enough to be copy
(I estimate none are larger than 4 bytes).