Added Delete List Item requests to the Lists collection

This commit adds new requests to the Lists collection in the form
of 'Delete List Item' requests.

Added Delete List requests to the Lists collection

This commit adds two new requests to the Lists collection,
enabling the ability to delete SharePoint lists based on a
specified GUID or title.

Updates duplicated request description

This commit updates the previously-duplicated description
for the 'Add Attachment File to List Item' request.

Added initial Postman collections

This commit contains the initial Postman collections, both a
collection of requests and a collection of environment variables.