Support enum keyword

[roblourens]

Have code like

<?php
enum Status : string
{
    case VALID = 'VALID';
    case NOT_FOUND = 'NOT_FOUND';
}

This is apparently a new PHP feature. enum is not colored. I think I expect a similar token to "class"

microsoft/vscode#142824

[KapitanOczywisty]

I'm working on it, PR should be up soon.