Improve handling of #[ attributes in php -a

`php -a` treats lines starting with `#` as comments when deciding if
the provided statement is valid.

So it passed `#[MyAttr]` to the parser after the user hits enter,
causing a syntax error for multi-line statements..

With this patch, the following snippet is parsed correctly

```
php > #[Attr]
php > function x() { }
php > var_export((new ReflectionFunction('x'))->getAttributes()[0]->getName());
'Attr'
```

Followup to GH-6085

Closes GH-6086

ext/readline/readline_cli.c

@@ -250,6 +250,10 @@ static int cli_is_valid_code(char *code, size_t len, zend_string **prompt) /* {{
 						code_type = dstring;
 						break;
 					case '#':
+						if (code[i+1] == '[') {
+							valid_end = 0;
+							break;
+						}
 						code_type = comment_line;
 						break;
 					case '/':