Add method to enable deleting a file without having access to its parent directory

[mkruisselbrink]

Before #55 it was possible to delete a file by just having access to a file handle. I.e. you could get a file handle from showSaveFilePicker, decide you don't want to save after all, and delete the file. Since now deletion requires access to the parent directory this is no longer possible (as pointed out in https://crbug.com/1114923), and I think we might want to bring back a "remove self" method, at least for file handles.

[pwnall]

This is going to be exciting to spec... we'll need to clarify that for files/dirs w/ multiple hardlinks/symlinks, the entry that gets deleted is the entry corresponding to the path that was used to obtain the handle.

[MartinZikmund]

This would be valuable not only for files but for directories as well.