games/libretro-shaders-slang: Update g20260322 => g20260402, improve port

• Reintroduce "Vulkan GLSL" into COMMENT and pkg-descr. It is official nomenclature and should be included.
• Use short hash for GH_TAGNAME. This follows the Porter's Handbook more closely.
• Install with COPYTREE_SHARE to ensure proper file ownership and permissions, and to not require the make binary. In this case do not extract unneeded build and CI files by excluding them via EXTRACT_AFTER_ARGS.

Commit log:
https://github.com/libretro/slang-shaders/compare/b256008...e42d2d3

PR: 294200
Sponsored by: UNIS Labs