Add JS execution capabilities to Client

[dunglas]

Allows to execute some JS in the context of the current page with $client->executeScript() and $client->executeAsyncScript():

Example:

// sync
$text = $client->executeScript('return document.querySelector(arguments[0]).innerText;', ['.myclass']);

// async
$text = $client->executeAsyncScript(<<<JS
setTimeout(function (parentArgs) {
    const callback = parentArgs[parentArgs.length - 1];
    const t = document.querySelector(parentArgs[0]).innerText;
    callback(t);
}, 100, arguments);
JS
    , ['.myclass']);