Append Kapt stats if file exists

[pettermahlen]

https://youtrack.jetbrains.com/issue/KT-55505

When running KAPT in a multi-module project, it's awkward to specify separate processor statistics dump files for each sub-module. This PR makes it so that previous statistics dumps are not overwritten, and adds a timestamp to make it easier to identify which run is which.

Ideally, there should be a way to identify which KAPT invocation is being reported on, but as far as I can tell, that information isn't available at the time the report file is created, and it seems like it might be too big a change.

I have not built this code locally, because I haven't been able to get the repository to build. The instructions in the README don't work for my M1. If the PR doesn't build, I was hoping that some kind soul might make whatever tweaks are necessary to make it mergeable. :)