fix(types): include the data attribute on the connect_error event

[lazerg]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behavior

connect_error is typed as plain Error, even though the client always attaches a data property to it (forwarded from the err.data set on the server-side middleware error). Accessing err.data from a listener is a type error unless you cast it yourself.

New behavior

connect_error is now typed as Error & { data?: any }, matching the ExtendedError shape already used on the server side. The internal @ts-ignore needed to attach .data in onpacket() is gone too, since the variable is properly typed.

Other information (e.g. related issues)

Fixes #5542

[darrachequesne]

Closing this PR since #5548 has been merged and supersedes these changes. Thanks!