Add file truncate() #11402

We don't have .truncate() and extending FAT files can be slow (and blocking), so high-frequency loggers need to preallocate oversized files, then later truncate them. This currently requires copying upon completion (slow + requires extra free space). Basically the same request as this upstream one:
micropython#4775

That one has a PR on it already, which looks pretty much how I'd expect: micropython#11500

Can we just grab that PR (or evangelize for MP to accept it or specify what exactly needs to change to get it accepted)?