add(sizing): grow configuration

Currently the 'grid' and 'mixed' examples are not working yet.

examples/demo.zig

@@ -61,7 +61,9 @@ pub fn main() !void {
             .padding = .vertical(2),
             .direction = .vertical,
         },
-        .size = .{ .y = 90 },
+        .size = .{
+            .dim = .{ .y = 90 },
+        },
     }, .{});
     try box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .light_green },
@@ -91,11 +93,15 @@ pub fn main() !void {
             .color = .light_blue,
             .sides = .all,
         },
-        .size = .{ .x = 100 },
+        .size = .{
+            .dim = .{ .x = 100 },
+        },
     }, .{}));
     try container.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .blue },
-        .size = .{ .x = 30 },
+        .size = .{
+            .dim = .{ .x = 30 },
+        },
     }, .{}));
     defer container.deinit(); // also de-initializes the children