doc(queryconfig): Allows configuring a different element query mechanism

[MatthiasKainer]

Provides the ability to specify a different element query mechanism other than querySelector and querySelectorAll.

An example of the usage with Georgegriff/query-selector-shadow-dom looks like this:

import { querySelectorAllDeep, querySelectorDeep } from 'query-selector-shadow-dom';
configure({
    queryElement: querySelectorDeep,
    queryAllElements: querySelectorAllDeep,
})

After this line, a query will also drill into the shadow dom of elements on the page

Why:

In our project, we are using a design system build with web components
(via stenciljs), and different projects in different frameworks (lit,
react, vue) want to use this library. The corresponding pr can be found
here:
testing-library/dom-testing-library#1054

[timdeschryver]

I haven' taken a look at the implementation PR, but I left a few remarks on the wording.