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/terminal.zig

@@ -1,5 +1,5 @@
 const std = @import("std");
-const code_point = @import("code_point");
+pub const code_point = @import("code_point");
 
 const Key = @import("key.zig");
 const Size = @import("size.zig");