Add addTint plugin

[YOHEMAL]

What changed

• Add the add_tint plugin using the current per-plugin repository structure.
• Add an English/Japanese About page with usage instructions and an author link.
• Register matching store metadata in plugins.json.
• Update to version 0.4.2 so grayscale conversion only affects UV regions used by selected faces.

Why

Minecraft Java model JSON supports multiple tintindex values, while Blockbench's default editing workflow does not provide a convenient way to assign and preview arbitrary indices. This plugin adds that workflow directly to the UV Editor, a dedicated panel, and the Filter menu.

The original grayscale implementation converted the entire texture referenced by a selected face. Version 0.4.2 maps each selected face's UV bounds to texture pixels so unrelated areas keep their original colors, including on layered and animated textures.

User impact

Users can assign or remove arbitrary tint indices, edit live preview colors, preview texture/color multiplication, and convert only selected-face UV areas to tint-friendly grayscale. Plugin UI follows Blockbench's Japanese language setting and falls back to English for other languages.

Validation

• npm run validate add_tint
• node --check plugins/add_tint/add_tint.js
• plugins.json JSON parse check
• git diff --check