Configurable path prefix

Problem/Motivation

Generating an alias at a top level path has a relatively high likelihood of conflicting with another existing path.

Proposed resolution

I considered whether the module should introduce a designated path prefix, like '/s/' or '/sa/', but couldn't decide what was most appropriate. Instead of deciding on path that I might not like in the future, I would prefer to introduce a configurable path prefix. While this does not prevent conflicting paths, it at least provides the capacity for introducing a pattern that might help mitigate such conflict.

Noteworthy that such configuration could exist per entity bundle, but I will make a separate issue for this if it ever feels like it would actually be a good idea.

User interface changes

This will require a config page for short alias, where the path prefix can be configured in a text input field. In the field description, it would probably be good to include a reminder to keep the path prefix short!