Forwards the isUndoing/isRedoing flags to the extension host.

[hediet]

This PR addresses #120617.

No time pressure, this can wait.

[jrieken]

This should maybe done like TextEditorSelectionChangeKind, e.g have a kind-property on the event which classifies how the event was triggered

[hediet]

I thought of some TextEditorSelectionChangeKind too, but that might give the wrong impression that the enum is exhaustive.

enum TextEditorSelectionChangeKind {
    Undo,
    Redo,
    Other // or unknown
}

Then users can check for if (event.changeKind === TextEditorSelectionChangeKind.Other).
If there ever will be the variant typeInput (or remoteEdit), that would break extensions.
Another boolean flag isTypeInput: boolean would not break extensions though, as they cannot really depend on its non-presence.

Alternatively, the enum has no Other variant but is stored in a field that can be undefined.
But then users could check if that field is undefined.

[jrieken]

Alternatively, the enum has no Other variant but is stored in a field that can be undefined.

That's how TextEditorSelectionChangeEvent.kind works - it can be undefined or set to a known state. The advantage of the enum is that it prohibits invalid states, e.g it doesn't allow for { isUndoing: true, isRedoing: true }