Add string type support: literals, concatenation, println, and if eval

- Implement string literal tokenization and parsing (lexer + parser)
- Add string concatenation with + operator in evaluator
- Add println keyword for printing with newline
- Add NODE_IF evaluation in VM
- Fix null terminator bug in string concat buffer

src/main.c

@@ -37,7 +37,7 @@ int main(int argc, char **argv) {
   eval(block, &env, allocPtr, 0, 1);
 
   printf("heapSize=%zu\n", allocPtr->size);
-  // JLANG_visualize(allocPtr);
+  JLANG_visualize(allocPtr);
 
   return 0;
 }