feat(terminal/osc12): define cursor color through style of cell that describes the cursor position

The `.default` color will reset the cursor color to the terminal's default color

examples/continuous.zig

@@ -156,7 +156,7 @@ pub fn main() !void {
     defer container.deinit();
 
     try container.append(try App.Container.init(allocator, .{
-        .rectangle = .{ .fill = .light_grey },
+        .rectangle = .{ .fill = .lightgrey },
         .size = .{
             .grow = .horizontal,
             .dim = .{ .y = 10 },
@@ -164,7 +164,7 @@ pub fn main() !void {
     }, input_field.element()));
 
     const nested_container: App.Container = try .init(allocator, .{
-        .rectangle = .{ .fill = .light_grey },
+        .rectangle = .{ .fill = .lightgrey },
     }, spinner.element());
     try container.append(nested_container);