Make `iter().foo()` and `await.foo()` suggestions configurable

[sanbox-irl]

Some people find these to be very helpful, but for me, I use auto-complete often to "survey" what I can or cannot do with a given type. Letting that auto-complete ever go beyond the current type, especially to a type I know very well (Ie, I know what methods are on Iter as an experienced dev), ends up cluttering my auto-complete window.

I suspect many/some/most/a few/a couple disagree with me, so perhaps this extension of the auto-complete could be configurable?

Note: I see that #18988 has been merged which will make the sorting better, but this is beyond sorting.

[Veykril]

We already have this for the self. prefix completions, so a config for these here as well sound reasonable.