chore: add GPLv3 (or later) licensing to repository

NOTE: lua/lazyload.lua is an MIT licensed file borrowed from another
user's repository. The license header in that file reflects this.

lua/plugins.lua

@@ -4,6 +4,21 @@
   └────────────────────────────────────────────────────────────────┘
 --]]
 
+-- Copyright (C) 2026 rootiest
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU General Public License as published by
+-- the Free Software Foundation, either version 3 of the License, or
+-- (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU General Public License for more details.
+--
+-- You should have received a copy of the GNU General Public License
+-- along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
 -- Catppuccin.nvim
 -- Eagerly load the colorscheme plugin.
 vim.pack.add({ { src = "https://github.com/catppuccin/nvim", name = "catppuccin" } })