Add Winston support to the logger

[unflxw]

It should be possible to turn an AppSignal logger (from the Logging API beta) into a Winston transport. Something like:

const winstonLogger = winston.createLogger({
    transports: [
        Appsignal.logger("groupname").winstonTransport()
    ]
})

User-requested; Intercom link (internal)

TO-DO:

• Create documentation page -- PR

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[unflxw]

Closed by mistake (see #823) -- reopened.