Add listener support to WindowsSettings

[russhwolf]

Probably using RegNotifyChangeKeyValue. It may take some effort to massage the Windows API to fit the pattern used on other platforms.

[russhwolf]

I've started prototyping stuff here. I've tried an implementation where each listener gets its own worker, which seems to work alright but I worry it might get heavy if someone tries setting lots of listeners, and it had some flakiness when I tried to load-test it. I think I want to look into an alternative where listeners all run on a single dedicated worker, but that gets more complicated to implement. Given timing and where the ecosystem is, there's a good chance this ends up being built in a way that only supports the new memory manager.

Ideas and contributions welcome.

[russhwolf]

This is the last thing I would like to have done before removing @ExperimentalSettingsImplementation from RegistrySettings.