Make widget theme overrides easier to merge across packages

Problem

Widget themes contributed by packages are merged through the widgets-themes configuration group. By default, nested arrays are replaced rather than recursively merged.

This makes a small application override unexpectedly replace the complete package theme definition. To preserve the package defaults, the application must either:

• add a RecursiveMerge modifier to the application runner, or
• manually load the vendor theme file and merge it with array_replace_recursive().

This differs from yiisoft/form, where the Bootstrap theme can be loaded and extended directly through ThemePath::BOOTSTRAP5_VERTICAL.

Proposal

Provide a supported helper or configuration mechanism for extending widget themes without requiring application-level merge modifiers or direct loading of vendor configuration files.

For example, widget theme groups could be recursively merged by default, or the package could expose a public theme extension API/path similar to yiisoft/form.

Context

This affects applications customizing the built-in bootstrap5 theme while retaining the package's other widget defaults.

Start with the widgets-themes configuration group and compare the proposed extension behavior with yiisoft/form's ThemePath::BOOTSTRAP5_VERTICAL. Determine which supported mechanism preserves package defaults while allowing application overrides, then verify that a bootstrap5 customization no longer requires an application merge modifier or direct vendor-file loading.