Can't use custom trigger type with default runner

## What problem does this solve or what need does it fill?
In the 0.17 update post, it said you can make custom trigger types. While technically true, because the increment_trigger_id is pub(crate) and this needs to be called for the default runner to execute a trigger, you will need to make a custom runner for them, and so it is functionally impossible to use app.add_observer() with a custom trigger type since this will use the default runner.

## What solution would you like?
make a way to call world.as_unsafe_world_cell().increment_trigger_id(); with the defered world passed to the trigger trait. So they can work with the default observer.

Start with the custom trigger path described around app.add_observer(), the default runner, increment_trigger_id, and the deferred world passed to the trigger trait. Trace how the default observer handles custom trigger types and determine how the trigger can access the needed world operation. Done means custom triggers work with app.add_observer() without requiring a custom runner.

Written by the indexing model from the issue text.