ItemList API improvements

Feature Request

Is your feature request related to a problem? Please describe.
ItemList currently serves as an ordered associative structure, with order determined by integer "positions". It would be convenient and more self-documenting if items could be inserted relative to other items. For instance:

• items.after('delete').add('something', hello)
• items.before('delete').add('something', hello)
• items.after('delete', 10).add() // specify a fallback priority
• items.after(['foo', 'bar'], 10).add() // specify multiple items to add after

Initially proposed in https://github.com/flarum/core/issues/1317

Describe the solution you'd like
We'd need to include a modified topological sort (like https://github.com/flarum/core/pull/2317), that also includes support for integer-based priority.