Add TempFileStorage

[skycoop]

I realized while implementing form-data-parser in a project that LocalFileStorage is a bad fit for apps that process uploads before persisting them elsewhere. I wanted something that wrote to a temporary directory that was removed at the end of the request handler and was only initialized if the form data contained a file. So I wrote TempFileStorage and wanted to contribute it back for others to use 😄

The biggest uncertainty I have about this change is the support for AsyncDisposable in the general node ecosystem.

[mjackson]

Whoa, this is new to me! I've never seen using before. Very cool.

[mjackson]

Thank you for the PR! This is very cool.

Since AsyncDisposable is so new, would you be open to publishing this package yourself? At least until we can determine if it's going to be generally useful for many people?

The whole idea with the FileStorage interface is that hopefully there will be many implementations for different storage backends, and people can just plug in whichever one they need for a given use case. I'd be happy to link to your package in the README to help others find it more easily.

[skycoop]

I have added use as a method to get automatic cleanup without await using and improved the documentation and tests on the class in the hopes it makes the class more generally useful. I can definitely publish this on my own if you prefer, but I believe that it's generic and useful enough to belong in the base package.