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

projects/test.j

@@ -0,0 +1,3 @@
+x = 5
+z = x > 2
+print x + z