fix(loader): Avoid setting/resetting window.onerror

[mydea]

This uses addEventListener instead of window.onerror to set the handler for the loader.
This way, we avoid infinite recursion when users also do that, e.g. right after loading the loader:

constoldOnError=window.onerror;window.onerror=function(){console.log('custom error');oldOnError&&oldOnError.apply(this,arguments);};window.doSomethingWrong();

Rundown of the problem:

The root cause for this issue was this new check we added:
https://github.com/getsentry/sentry/pull/47311/files#diff-02afb4c480e9e9c8bcc2f5fd8096291fd71d113cc2d50139624c6d890fed6e88L54

So where did this cause issues?
Imagine following setup and order of users' Website

-> Loader script
-> Other script that overwrites window.onError
-> throw an unhandled Error

What was happening is that because of the if

if(_window[_onerror]&&_window[_onerror].__SENTRY_LOADER__){_window[_onerror]=_oldOnerror;}

this is invoked whenever we the full SDK was loaded but not initialized yet.

Previously it was this:

_window[_onerror]=_oldOnerror;

Since initially window.onError is null we caused an endless loop of re-calling the Loader onError

_window[_onerror]=function(){queue({e: [].slice.call(arguments),});if(_oldOnerror){_oldOnerror.apply(_window,arguments);}};

on the other, the external onError handler, caused the site to slowly run out of memory.
Now we use event listeners instead to not have to deal with overwriting onError.

In the old version where we still dealt with onError callback - we also caused a bug where we set the handler back to null regardless if a second one was registered in the meantime.

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[codecov]

Codecov Report

Merging #47311 (db6c72c) into master (0e5a5e5) will decrease coverage by 7.80%.
The diff coverage is n/a.

Additional details and impacted files

@@ Coverage Diff @@## master #47311 +/- ##
==========================================
- Coverage 80.65% 72.85% -7.80% 
==========================================
Files 4752 4723 -29 Lines 201025 200640 -385 Branches 11648 11648 ==========================================
- Hits 162127 146177 -15950 - Misses 38640 54205 +15565 
Partials 258 258 

see 847 files with indirect coverage changes