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/str.j

@@ -0,0 +1,12 @@
+x = 0
+while x < 10:
+    x = x + 1
+
+if x > 9:
+    println "fin"
+
+
+x = "a"
+y = x * 1
+z = y + 2
+println "a" + z