Add a register_error_handler config option

[HypeMC]

My second attempt at getsentry/sentry-php#1439 .

Pretty much what was described in #337 (comment) , eg:

sentry:
    dsn: '%env(SENTRY_DSN)%'
    register_error_listener: false
    register_error_handler: false

monolog:
    handlers:
        sentry:
            type: sentry
            level: !php/const Monolog\Logger::ERROR
            hub_id: Sentry\State\HubInterface

[HypeMC]

This isset was unnecessary since the configuration always returns an array.

[cleptric]

Thanks for the contribution! I'll take a look in January, as we're wrapping up things for this year.

[cleptric]

Overall, this LGTM! Thanks for your work on this 🙂

[HypeMC]

FYI, I've opened a PR for this option in the Symfony recipes repo symfony/recipes-contrib#1485