Add GPL-3.0-or-later license headers to source files

- Updated all relevant source files to include the new license header.
- Changed the default license in the configuration from MIT to GPL-3.0-or-later.

src/utils/text.rs

@@ -1,3 +1,20 @@
+// DCR — Cargo-like C/C++ project manager.
+//
+// Copyright (C) 2026 Dexoron (Bezotechestvo Vladimir) <main@dexoron.su>
+//
+// 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/>.
+
 #[allow(dead_code)]
 pub const RESET: &str = "\x1b[0m";