[OndraM]

This PR adds Shadow DOM support. Fixes #1003 .

Shadow DOM is essential part of Web Components, as it encapsulates the component DOM and styles. This component isolation however mean one cannot access elements which are encapsulated in shadow DOM of a component with usual $driver->findElement() and $driver->findElements() method. You will basically not by able to find those elements in the default DOM.

Because of this, W3C WebDriver added support for endpoints specifically made for accessing Shadow DOM elements.

So far, this has been implemented in Chromedriver and partially in Geckodriver.

Example usage:

// consider having web component <custom-element>...</custom-element> with its inner DOM

// first, locate the element of the Web Component, which is part of the default DOM
$element = $this->driver->findElement(WebDriverBy::cssSelector('custom-element'));

// locate Shadow Root, which is the base for accessing encapsulated elements
$shadowRoot = $element->getShadowRoot();

// ShadowDom instance now provide findElement() and findElements() method, which will target only elements inside this Shadow DOM
$elementInShadow = $shadowRoot->findElement(WebDriverBy::cssSelector('input'));

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• Add documentation to wiki, see https://github.com/php-webdriver/php-webdriver/wiki/Example-command-reference#shadow-dom