add(sizing): grow configuration

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

examples/elements/scrollable.zig

@@ -98,15 +98,21 @@ pub fn main() !void {
     }, .{});
     try top_box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .light_green },
-        .size = .{ .y = 30 },
+        .size = .{
+            .dim = .{ .y = 30 },
+        },
     }, .{}));
     try top_box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .light_green },
-        .size = .{ .y = 5 },
+        .size = .{
+            .dim = .{ .y = 5 },
+        },
     }, element));
     try top_box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .light_green },
-        .size = .{ .y = 2 },
+        .size = .{
+            .dim = .{ .y = 2 },
+        },
     }, .{}));
     defer top_box.deinit();
 
@@ -123,6 +129,9 @@ pub fn main() !void {
             .direction = .vertical,
             .padding = .vertical(1),
         },
+        .size = .{
+            .dim = .{ .y = 30 },
+        },
     }, .{});
     try bottom_box.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .grey },