NpmTask workingDir should be DirectoryProperty

[wakingrufus]

NpmTask's workingDir property is currently a RegularFileProperty but it probably should be a DirectoryProperty similar to how NodeTask's workingDir property is.

I can implement this, if you wish, but I am not sure how you would want to handle breaking compatibility with existing code.

[deepy]

Next version is going to be a major so feel free to send a PR :-)
And no tests breaks with the change either, so this was potentially just an oversight when we moved to providers and properties 4 years ago