feat(icons): embed minimal nvim-web-devicons subset + compat shim

[M4ikel79]

• Add a minimal embedded icon provider (lua/volt/icons.lua) with caching and a lazy loader for a full icon set.
• Include a small extracted subset for demonstration in lua/volt/icons/full.lua.
• Provide a compatibility shim for plugins expecting nvim-web-devicons in lua/volt/compat/devicons.lua.
• Add a script scripts/extract_icons.lua to generate a complete icons/full.lua from an installed nvim-web-devicons.
• Expose icons API and a helper M.get_file_icon in lua/volt/init.lua.
• Notes: By default Volt stays lightweight; call volt.icons.load_full_icons() to load the full set (from icons/full.lua) or use the extraction script to regenerate the full set from your nvim-web-devicons installation.