listView: document `on:itemLoading` and `on:loadMoreItems` events

[jasongitmail]

Only the on:itemTap event is documented on the Svelte Native website currently.

It'd be good to document these other events too, especially because Svelte Native's property ofon:itemLoading is inconsistent with on:itemsLoading (plural) mentioned in the NativeScript docs.

Versions:

"@nativescript/core": "~8.6.2",
"svelte-native": "~1.0.15"