add(container/layout): sizing option percentage

examples/container.zig

@@ -33,8 +33,8 @@ pub fn main() !void {
         .layout = .{
             // .gap = 1,
             .sizing = .{
-                .width = .{ .fixed = 100 },
-                .height = .{ .fixed = 50 },
+                .width = .{ .percent = 50 },
+                .height = .{ .percent = 50 },
             },
             .padding = .all(5),
             .direction = .horizontal,