Added tons of opcode documentation, changed license to ISC and a lot of file refactoring.

src/vm/core/dup.c

@@ -1,3 +1,35 @@
+/**
+ * This file is part of the Fun programming language.
+ * https://hanez.org/project/fun/
+ *
+ * Copyright 2025 Johannes Findeisen <you@hanez.org>
+ * Licensed under the terms of the ISC license.
+ * https://opensource.org/license/isc-license-txt
+ */
+
+/**
+* @file dup.c
+ * @brief Implements the OP_DUP opcode for duplicating the top stack value in the VM.
+ *
+ * This file handles the OP_DUP instruction, which duplicates the top value on the stack.
+ * The duplicated value is pushed back onto the stack.
+ *
+ * Behavior:
+ * - Duplicates the top value on the stack.
+ * - Pushes the duplicated value onto the stack.
+ *
+ * Error Handling:
+ * - Exits with an error if the stack is empty.
+ *
+ * Example:
+ * // Bytecode: OP_DUP
+ * // Stack before: [42]
+ * // Stack after: [42, 42]
+*
+ * @author Johannes Findeisen
+ * @date 2025-10-16
+ */
+
 case OP_DUP: {
     if (vm->sp < 0) {
         fprintf(stderr, "Runtime error: stack underflow for DUP\n");