Update vfs to implement io/fs's fs.FS interface

[ThisaraWeerakoon]

Fixes #163

Add Stat() method to File interface.
Add Open() method to Location interface
Add Mocks for testing
Implementation for each backend

[ThisaraWeerakoon]

Hi @funkyshu
Would you have some time to review this PR? Your feedback would be greatly appreciated. Thank you!

[funkyshu]

@ThisaraWeerakoon,
Thanks for this awesome contributions. In this case, I'm not 100% We actually want this as part of vfs (yes, I'm keenly aware that I added the feature request). The current interface is quite bloated already and while I love the idea of being compliant with a std interface, I don't love the ever-growing composition in io.FS (readdir, readfile, sub, etc). Have said all of that, I look forward to looking at your implementation across the various backends. As stated elsewhere, time is not on my side at the moment but I'll try to get to this in the next couple of weeks. Thanks again for all your contributions.