config: make window-padding-color default to "background"

Unfortunately this is a failed experiment. The idea works in many
scenarios, but there are too many valid cases where it confuses people
or results in a very subjective experience. I'm going to revert this
back to "background" to act like other terminals, but the feature
remains available through config.

src/config/Config.zig

@@ -806,7 +806,7 @@ keybind: Keybinds = .{},
 /// * The nearest row contains a perfect fit powerline character. These
 ///   don't look good extended.
 ///
-@"window-padding-color": WindowPaddingColor = .extend,
+@"window-padding-color": WindowPaddingColor = .background,
 
 /// Synchronize rendering with the screen refresh rate. If true, this will
 /// minimize tearing and align redraws with the screen but may cause input