feat: add triple-quoted multiline string literals (issue 12)

Support """...""" syntax for multiline strings with:
- Automatic indent stripping (based on minimum indentation)
- Leading newline after opening """ is skipped
- Trailing whitespace-only line before closing """ is stripped
- String interpolation ({expr}) support
- All escape sequences supported
- Formatter outputs multiline strings for strings containing newlines

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.rs

@@ -3925,6 +3925,49 @@ c")"#;
         assert_eq!(eval(source).unwrap(), r#""literal {braces}""#);
     }
 
+    #[test]
+    fn test_multiline_string() {
+        let source = r#"
+            let s = """
+                hello
+                world
+            """
+            let result = String.length(s)
+        "#;
+        // "hello\nworld" = 11 chars
+        assert_eq!(eval(source).unwrap(), "11");
+    }
+
+    #[test]
+    fn test_multiline_string_with_quotes() {
+        // Quotes are fine in the middle of triple-quoted strings
+        let source = "let s = \"\"\"\n    She said \"hello\" to him.\n\"\"\"";
+        assert_eq!(eval(source).unwrap(), r#""She said "hello" to him.""#);
+    }
+
+    #[test]
+    fn test_multiline_string_interpolation() {
+        let source = r#"
+            let name = "Lux"
+            let s = """
+                Hello, {name}!
+            """
+        "#;
+        assert_eq!(eval(source).unwrap(), r#""Hello, Lux!""#);
+    }
+
+    #[test]
+    fn test_multiline_string_empty() {
+        let source = r#"let s = """""""#;
+        assert_eq!(eval(source).unwrap(), r#""""#);
+    }
+
+    #[test]
+    fn test_multiline_string_inline() {
+        let source = r#"let s = """hello world""""#;
+        assert_eq!(eval(source).unwrap(), r#""hello world""#);
+    }
+
     // Option tests
     #[test]
     fn test_option_constructors() {