parser: support nullable types

phpast/samples/type-strings.php

@@ -2,4 +2,8 @@
 
 function foo(string $bar) {
 
+}
+
+function bar(?string $baz) {
+    
 }

trunk_parser/src/parser/mod.rs

@@ -48,6 +48,11 @@ impl Parser {
                     self.next();
                     return Ok(t);
                 },
+                TokenKind::Question => {
+                    self.next();
+                    let t = self.type_string()?;
+                    return Ok(Type::Nullable(Box::new(t)));
+                },
                 _ => todo!()
             }
         };