add(element/radio-button): RadioButton Element implementation

This can be used to visualize the values of `bool`'s, which is relevant
when creating form's based on `struct`'s automatically.

build.zig

@@ -11,6 +11,7 @@ pub fn build(b: *std.Build) void {
         button,
         input,
         progress,
+        radio_button,
         scrollable,
         // layouts:
         vertical,
@@ -62,6 +63,7 @@ pub fn build(b: *std.Build) void {
                 .button => "examples/elements/button.zig",
                 .input => "examples/elements/input.zig",
                 .progress => "examples/elements/progress.zig",
+                .radio_button => "examples/elements/radio-button.zig",
                 .scrollable => "examples/elements/scrollable.zig",
                 // layouts:
                 .vertical => "examples/layouts/vertical.zig",