feat: Determine Electron process from minidump metadata

[timfish]

• Closes Parse minidumps to determine which process they came from #1047

As described in the above issue:

Currently we read minidumps from the crashpad directory when we are notified by Electron that a renderer or child process has crashed. We assume that all the minidumps we find correspond the the Electron event. All minidumps we read at startup are assumed to be main process crashes because they cause the process to exit immediately.

Although these assumptions are usually correct, I've personally seen events recorded incorrectly and so have other users (#1045).

Rather than fully parse the minidumps as described in the above issue (ie. header + enumerate the streams), we just search for the process_type key in the minidump file and pick out the string that follows. This simplified approach is reasonable because Electrons Crashpad is configured to emit minimal minidumps (250-400kb).

If we can't determine the process type from parsing the minidump, we have a new event.process tag of unknown. In most cases, these will be for child processes and I've added a test to confirm that if these are found at startup, they are correctly categorised.