WIP: add Container type with corresponding Properties configuration

The configuration of the `Container` types is very much inspired by
[clay](https://github.com/nicbarker/clay).

src/size.zig

@@ -1,8 +1,8 @@
 pub const Size = @This();
 
 pub const Position = struct {
-    col: u16,
-    row: u16,
+    col: u16 = 0,
+    row: u16 = 0,
 };
 
 anchor: Position = .{},