Add support for readonly constructor properties

[andrewnicols]

public function __construct(
    public readonly int $foo,
    protected readonly int $bar,
) {}

The readonly keywords were being detected as types.

[andrewnicols]

Okay, this should now be sorted.

PHP 8.0 and earlier lack the T_READONLY const so I had to refactor for that, and then realised that correct commenting for promoted properties also broke the sniff.

[stronk7]

I assume that CodeSniffer handles these? Just thinking in advance for anything we may have there related.