Add block_data() and block_export() functions

[seb-jean]

Summary

This PR adds two new Twig functions that allow blocks to export structured data alongside their rendered HTML output:

• block_export(data): called inside a block to store structured data on CoreExtension via a side-channel. The block continues to render HTML normally.
• block_data(name): renders the block (HTML output is discarded) and returns the data exported via block_export(). Returns [] if the block does not call block_export().

Use case

Twig Components (<twig:> syntax) use the spread syntax ({{ ...hash }}) which requires an associative array — not an HTML string. Today, blocks like widget_attributes in Symfony's form themes render HTML attribute strings, making them incompatible with the <twig:> parser.

With block_data() / block_export(), a Symfony form theme block can export its attributes as a hash while still rendering HTML for backward compatibility:

{% block attributes %}
    {%- set attrs = {} -%}
    {%- for attrname, attrvalue in attr -%}
        {%- if attrvalue is same as(true) -%}
            {%- set attrs = attrs|merge({(attrname): attrname}) -%}
        {%- elseif attrvalue is not same as(false) -%}
            {%- set attrs = attrs|merge({(attrname): attrvalue}) -%}
        {%- endif -%}
    {%- endfor -%}
    {%- do block_export(attrs) -%}
    {%- for attrname, attrvalue in attrs -%}
        {{- ' ' ~ attrname }}="{{ attrvalue }}"
    {%- endfor -%}
{% endblock %}

A Twig Component form theme can then use:

<twig:Input type="{{ type }}" {{ ...block_data('widget_attributes') }} />

Implementation

• block_data() uses a parser_callable to compile to a BlockDataExpression node (mirrors the block() / BlockReferenceExpression pattern)
• BlockDataExpression compiles to $this->unwrap()->renderBlockData(...)
• Template::renderBlockData() calls renderBlock() then returns the exported data from CoreExtension::getExportedBlockData()
• The exported data is consumed on read (reset to null), preventing leaks between calls

Tests

4 test fixtures covering: basic usage, block override, block parent, and missing export fallback.

Closes #4919
Related: symfony/symfony#65713