[TwigComponent] Simplify macros usage

[javiereguiluz]

As explained in the docs (https://symfony.com/bundles/ux-twig-component/current/index.html#using-macros-in-components) you cannot use Twig macros inside components unless you import the full template explicitly:

{% macro message_formatter(message) %}
    <strong>{{ message }}</strong>
{% endmacro %}

<twig:Alert>
    {# ❌ this won't work #}
    {% from _self import message_formatter %}

    {# ✅ this works as expected #}
    {% from 'path/of/this/template.html.twig' import message_formatter %}

    {{ message_formatter('...') }}
</twig:Alert>

Using _self. is so convenient for calling macros defined in the same template that I think we should improve this to avoid the template import. Some proposals:

1. Allow to use _self via outerScope:

<twig:Alert>
    {{ outerScope._self.message_formatter('...') }}
</twig:Alert>

2. Add a new outerMacros variable with the macros of the current template, similar to the existing outerBlocks:

<twig:Alert>
    {{ outerMacros.message_formatter('...') }}
</twig:Alert>

Thanks!