Add warning variant for vaadin-button

[TatuLund]

Describe your motivation

As Vaadin 24.1 has defined now Warning colors, for sake of consistency vaadin-button should have a warning variant. This kind of button is needed e.g. in dialogs which have a warning nature.

Describe the solution you'd like

I used these styles in my test app

:host([theme~='warning']) {
    background-color: var(--_lumo-button-primary-background-color, var(--lumo-warning-color));
    color: var(--_lumo-button-primary-color, var(--lumo-warning-contrast-color));
    font-weight: 600;
    min-width: calc(var(--lumo-button-size) * 2.5);
}

Describe alternatives you've considered

No response

Additional context

No response

[TatuLund]

See also request for ConfirmDialog theme variants: #5912