Error handler as singleton and with listeners

[Jean85]

Sorry for opening a new PR, but this is a new approach to solve #746, different from #759.

In here, as discussed together, we use the integrations to attach two listeners to the error handler, moving away the issue of error level filtering from that class.

I like this approach very much:

• it's cleaner
• tests are better and cleaner
• the end user can use the listeners to attach custom reporting
• the end user can even use the default integrations to decide which auto-reporting to enable (errors or exceptions)

[HazAT]

We should not forget to update docs: https://github.com/getsentry/sentry-docs/blob/master/src/collections/_documentation/platforms/php/default-integrations.md

[HazAT]

Otherwise, this PR really looks good, like the approach. I will close my PR :)
Close #759

[ste93cry]

Can you please also add an entry to the CHANGELOG.md file?

[RobinHoutevelts]

Was just about to complain that \Sentry\ErrorHandler::handleError also called the previousExceptionHandler with the handmade \ErrorException.

I'm not sure this PR also fixes that but I'll check it out when I find the time.

Either way this is a nice improvement 👍

[Jean85]

We completely changed the approach there, splitting in two separate listeners, so that should be fixed!

[ste93cry]

It's not, @RobinHoutevelts is right in saying that we're still calling the $previousExceptionHandler in the handleException method regardless of the type of exception. We should investigate if this is an unintended behavior and in case fix it

[Jean85]

But that's no longer a problem, because we're no longer invoking the handleException from handleError nor handleFatalError methods, thanks to the listeners.

I broke that (not ideal) chain with this PR.

[ste93cry]

You're right, sorry for the mistake then