Figma variants support

Added a new component: ComponentSet. It allows defining component variants.

Example of usage:

<ComponentSet name="Button">
    <Component name="variant=Primary">
        <PrimaryButton text="Primary" />
    </Component>
    <Component name="variant=Dangerous">
        <DangerousButton text="Dangerous" />
    </Component>
    <Component name="variant=Secondary">
        <SecondaryButton text="Secondary" />
    </Component>
</ComponentSet>