All of the internal protocol handlers are restricted to `{.raises.}` of
`[CancelledError]`. However, the `LPProtoHandler` type is part of the
public interface, and example / test code raises more errors than just
`[CancelledError]`. The caller is aware of that and `CatchableError` is
caught.

To allow annotating the internal handlers with the proper `{.raises`.}
annotation, support for an extra `LPProtoHandler2` is added. This is
backward compatible as the old `LPProtoHandler` is still allowed.
Examples still compile fine. There is one test that needs a slight
adjustment as it accesses the internal `handler` field directly. That
field needs to be renamed so that the `template` is used instead.

Eventually, `LPProtoHandler` may be phased out, with appropriate notices
to users who define custom handlers and the migration requiring errors
to be handled inside the handler instead of raising them. At this time,
such a deprecation is not yet applied, especially while the internal
logic is still relying on the previous handler flow.