Improve cancellation support in LaunchSettingsProvider

The `LaunchSettingsProvider` handles concurrent modifications by queuing them up in a `SequentialTaskExecutor` where they are processed one at a time. Once a `Task` is queued up there is no way of cancelling it, with the exception that disposing the `LaunchSettingsProvider` will cancel all pending `Task`s.

We should looking into updating `LaunchSettingsProvider` (and `ILaunchSettingsProvider`) to accept and respect `CancellationToken`s.