Restore inheritance gutter markers on Twig blocks

[Soner (shyim)]

Summary

Fixes #307

The Symfony plugin used to show inheritance gutter markers on Twig blocks. After 0.1.0 replaced that integration with native sw_extends navigation, the markers disappeared.

This restores them as a plugin-owned RelatedItemLineMarkerProvider on {% block name %}:

• Overrides (up the chain): shown when the template extends another one and the same block exists upstream. Clicking navigates to those blocks, nearest parent first.
• Overridden (down the chain): shown when other templates extending this one define the same block. Clicking navigates to those overrides.

Sibling templates that all write @Storefront/... are not treated as children of each other. Downstream resolution uses a reverse sw_extends index (ShopwareTemplateExtendsIndex) plus the existing block index, so deciding whether to show a marker does not load other templates' PSI.

TwigUtil.getExtendsChainPaths is now cached per file, since every block of a template walks the same chain.

Tests

TwigBlockMarkerTest covers:

• core template only marked as overridden
• plugin in the middle of the chain marked in both directions
• last override (theme) only marked as overriding, not as a child of its siblings
• a block with no related blocks has no marker
• overrides marker navigates up the whole chain, nearest parent first
• overridden marker navigates to every override