Update customized built-in element example to match its source

[quepasaquepasa]

Fixes #44952

The ExpandingList snippet in the "Customized built-in elements" section was still the pre-mdn/web-components-examples#81 version, which:

• assigns to self in the constructor (self = super()), clobbering the global {{domxref("Window.self")}}. Even scoped to a module variable this would be wrong, since it only ever points at the most recently constructed instance, so a page with more than one expanding list would have every callback operate on the last one.
• declares a constructor that only calls super(), which the implicit constructor already does.

This syncs the snippet with the current source: no constructor, this instead of self, for…of over the live NodeLists instead of Array.from(...).forEach(...), and optional chaining on childText.parentNode.

The surrounding prose is updated to match — it no longer says "most of the code" is in connectedCallback() (now all of it is), and briefly explains why there's no constructor and what this refers to, which is the explanation the issue asked for.