feat(element/alignment): alignment Element implementation

You can now align a `Container` using the Alignment `Element` similar to
how you make a `Container` scrollable. For usage details please see the
example and the corresponding tests.

build.zig

@@ -6,6 +6,7 @@ pub fn build(b: *std.Build) void {
         all,
         demo,
         // elements:
+        alignment,
         button,
         input,
         scrollable,
@@ -54,6 +55,7 @@ pub fn build(b: *std.Build) void {
             .root_source_file = b.path(switch (@as(Examples, @enumFromInt(e.value))) {
                 .demo => "examples/demo.zig",
                 // elements:
+                .alignment => "examples/elements/alignment.zig",
                 .button => "examples/elements/button.zig",
                 .input => "examples/elements/input.zig",
                 .scrollable => "examples/elements/scrollable.zig",