Adding support to send screenshot to sentry event logging

[mikeesouth]

I've tested that this works for Android and I'm assuming iOS too. Sentry does not support screenshots in web and neither does catcher_2. Because of this we can use the regular dart:io File to parse the screenshot data and send it to Sentry.

[ThexXTURBOXx]

Thank you for the contribution!
However, I am currently sick and not able to work on this for the coming days, sorry. I suspect that the current code will not compile on Web due to the dart:io import.
It might be nice if you could test that. Else, I will test when I am healthy again :)

[mikeesouth]

Happy to help out! Good question but as far as I know the dart:io import in itself is not a problem for web platforms, using (most of the) stuff in the dart:io in web is a problem. I added a comment about this and also have a condition on !kIsWeb. I've tested this in web projects and it works just fine 👍

No rush in merging this for my sake even though I would appreciate it being added to the catcher_2 library of course. I will be using my own fork until catcher_2 has support for cleaning up screenshots after they have been submitted to all the registered handlers. Maybe I will submit that, maybe you have ideas or maybe some other kind soul out there. I'm worried that a device with multiple errors could quickly build up quite much storage space in form of error screenshots. Even if I use a temporary folder for the screenshots, I feel that it's important to cleanup the screenshot after it's been submitted. I mentioned this in #19 but my quick fix is to delete the screenshot in the sentry_handler and that will remove the file from any subsequent handlers in the list so it's not a good general solution. I added a couple of ideas of a general solution but I didn't want to make the core methods async and I didn't really have time to investigate more (at least not right now).

I hope you'll get well soon and good job with updating and maintaining this great library 👍

[ThexXTURBOXx]

Thank you very much for the exaplanation! :)
I have pushed this to pub.dev as version 1.2.0.
For the other improvements you mentioned, feel free to open another PR!