✨ Allow sorting middleware

[alecgibson]

At the moment middleware is executed in the order that it is registered.

This is fine if middleware is only registered in a single place, but in distributed setups, there may be multiple places in code (possibly across repos) that consumers may want to register middleware.

In order to allow some distributed way of defining an execution order of middlewares, this is a non-breaking change that lets consumers define an order for their middleware when registering:

backend.use('commit', fn, 10);

This order will be used to sort the middlewares, such that higher order execute after middleware with lower order. Ties are broken on registration order (the current behaviour), since JavaScript's .sort() is stable.

The default value for order is 0, so if consumers want to run middleware before other middleware that has not specified an order, they will need to set a negative order (which is legal):

backend.use('commit', fn, -10);