Add reordering functionality to lists

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
Lists are often reorderable, and list ordering via drag-and-drop is a rather complex and often-used feature that would fit better in this library instead of being reimplemented each time.

**Describe the solution you'd like**
An event, `onreordered`/`reordered`, that's sent to the list (or maybe the list item) for each movement. The event would contain information about which `mwc-list-item` moved, and the index it came from and moved to. Ideally, a parameter on `mwc-list-item` (`reordering`?) would be set, so custom `mwc-list-item` replacement components would be able to change their style and/or layout when being reordered.

**Describe alternatives you've considered**
Building custom list and list-item components to implement this behavior.

**Additional context**

https://material.io/components/lists/#behavior
#525 is potentially related?

Start by reviewing the existing list and list-item components, then compare their behavior with the linked Material Design lists guidance and related issue #525. The issue does not name specific files or tests; done would require an agreed reordering interaction and event/API behavior for moved items and their indices.

Written by the indexing model from the issue text.