Add pre-change notifications to sources

### What is the problem or limitation you are having?

At present, change notifications on a source are all "post change" - the notification is sent after the change has taken effect.

#4202 has highlighted that there is a need for "pre-change" notifications as well so that representations can accommodate changes *before* the change occurs. There are also opportunities to optimize other widgets on other platforms (e.g., DetailedList on macOS) if pre_notifications are available.

### Describe the solution you'd like

Every notification that is generated by a Source (ListSource, TreeSource or ValueSource) should have a corresponding "pre_" analog - so:

* `pre_insert` is issued just before an insertion occurs
* `pre_change` is issued just before a change occurs
* `pre_delete` is issued just before a deletion occurs
* `pre_clear` is issued just before a deletion occurs.

These "pre" notification should be given the same context as their corresponding "post" notifications (e.g. `index` and `item` for a `pre_delete` notification).

The existence of these new notifications should *not* require any additional changes to existing code. An existing listener *can* implement a `pre_*` notification handler, but is not *required* to implement that handler.

### Describe alternatives you've considered

Status quo - continue to only have post-notifications, and accommodate them.

### Additional context

_No response_

Start by locating the ListSource, TreeSource, and ValueSource implementations and tracing their existing post-change notification paths. Check how context such as index and item is passed to listeners. Done means each post notification has a corresponding pre_ notification with equivalent context, while existing listeners continue to work unchanged.

Written by the indexing model from the issue text.