[question] Exposed members on Polka class

Nobody has claimed this yet.

As part of some migrations, i've noticed myself needing the following members from `Polka`:

- `Polka#handler` - so i can bind it to `request` myself rather than calling `listen`
- `wares` - i want to create a new `Polka` which has the same middleware stack as another. for now i've literally set `b.wares = a.wares` but maybe we should have a method for this?
- `onError` - i want to set the error handler _after_ instantiating `Polka`

basically, can i rely on these 3 things being part of the public interface? (i.e. you won't remove them without a new major)

and if not, what could we introduce to cover the same use cases in a supported way?

Start by reading the Polka implementation and its public API documentation, focusing on handler, wares, onError, and listen. Determine whether these members are intended to be supported and document the decision or define supported alternatives for binding a handler, reusing middleware, and setting an error handler after instantiation.

Written by the indexing model from the issue text.