[FEATURE] Add :nbsp: text role for non-breaking spaces

[CybotTM]

Summary

Implements the :nbsp: text role which produces a non-breaking space ( ). This provides an explicit alternative to the ~ syntax (e.g., a~b) for inserting non-breaking spaces.

Usage:

This is a non breakable space: a :nbsp:`ignored` b

The content inside the role is ignored; the role simply produces a non-breaking space.

Background

• The functional test for :nbsp: was imported from doctrine/rst-parser in commit 77e1f40c
• The infrastructure (WhitespaceInlineNode and nbsp.html.twig template) already existed
• Only the text role class was missing, so the test was marked as SKIP

Changes

• Add NbspTextRole class that returns WhitespaceInlineNode
• Register the text role in the service configuration
• Remove SKIP marker from the functional test

Test plan

• Run functional test nbsp-role - now passes
• PHPCS - no errors
• PHPStan - no errors
• Unit tests - all pass
• Functional tests - 9 incomplete (was 10)

[CybotTM]

Alternative: Remove the test instead?

After implementing this, I'm questioning whether the :nbsp: role is actually needed:

The ~ syntax already works:

a~b  →  a b

The :nbsp: role is more verbose and requires dummy content:

a :nbsp:`x` b  →  a   b

Options

1. Merge this PR - Keep the role for feature parity with doctrine/rst-parser
2. Remove the test instead - The ~ syntax is sufficient, less code to maintain
3. Keep as SKIP - Leave it as a known limitation

Question for maintainers

The test was imported from doctrine/rst-parser (commit 77e1f40c), but the implementation was never added. Was this intentional (:nbsp: role not wanted) or an oversight?

If the :nbsp: role isn't needed, I'm happy to close this PR and submit one that removes the tests/Functional/tests/nbsp-role/ directory instead.

[jaapio]

Thanks for this PR, this test slipped through our reviews.

[jaapio]

Thanks for this fix!