Add counter to dotnet watch

Make it possible to detect from the inner process how many times the process has been restarted.

Ideas:
set the environment variable DOTNET_WATCH_ITERATION to an integer, starting at 0

Example:
```
> dotnet watch run
Starting: dotnet run, DOTNET_WATCH_ITERATION=0
(Change to file, dotnet-run is killed)
Retarting: dotnet run, DOTNET_WATCH_ITERATION=1
```

cc @danroth27 