Is your feature request related to a problem? Please describe.
Currently it's not possible for the user to reorder tags or non-hierarchical taxonomy terms in the Gutenberg editor. While this is not usually important for tags, other taxonomies may have a desired order other than alphabetical (for example, I'm making a coauthor plugin where author order is important). Since WordPress saves tags and non-hierarchical terms in the order it receives them, control should be given to the user to allow this.

This work probably goes hand-in-hand with #6688.

Describe the solution you'd like
The ability to reorder tags using some sort of drag-and-drop interface. JQuery has such a module to easily facilitate this, and it seems React does too.

If the developers don't want this feature to be enabled by default, consider adding it as an optional flag in register_taxonomy.

Describe alternatives you've considered
The only way to reorder terms is currently to delete the terms and add them back in the order you want. The terms are then submitted in this order, and WordPress saves them in this order, but then due to the issue described in #6688 the terms are retrieved in alphabetical order when the post is edited again.