feat(itemSelection): add support for items getter

[43081j]

Sometimes, we need to retrieve the item list dynamically rather than from a constant array we passed at constructor time.

This adds support for passing a function as the items array (() => T[]).

For example:

let items = [];
new ItemSelectionController(this, items);

// re-assign items. this wouldn't affect the controller previously. it
// will still see the original array
items = [a, b];

// instead, we can do this:
new ItemSelectionController(this, () => items);