Add OS specific settings (#35756)

Release Notes:

- Settings can now be configured per operating system with the new
top-level fields: `"macos"`/`"windows"`/`"linux"`. These will override
user level settings, but are lower precedence than _release channel_
settings.

crates/theme/src/settings.rs

@@ -867,6 +867,7 @@ impl settings::Settings for ThemeSettings {
             .user
             .into_iter()
             .chain(sources.release_channel)
+            .chain(sources.operating_system)
             .chain(sources.profile)
             .chain(sources.server)
         {