[docs] Describe 'Add the capability to perform testing in multiple browser windows' feature

[miherlosev]

The description should contain the following things:

• the feature description should be marked as BETA.
• how to run TestCafe in the Multiple windows mode (Add the capability to perform testing in multiple browser windows #912 (comment)).
• execution context on open/close windows: TestCafe automatically switches the execution context to the new opened window or to the parent of closed window. It allowing customers to avoid adding the additional 'switchToWindow' action.
• open/close/switch API. See the list of supported API methods here.
• the current restricts: Videos and screenshots are not supported for child windows, Cloud browsers are not supported
• examples (see them here and here)

[AlexKamaev]

#5277

API description:

1. getCurrentWindow(): unknown
   • returns object of the unknown type which contains information about currently active window.
2. openWindow(url: string)
   • opens new window from current. New window now is active window. Previous window is parent window.
3. switchToWindow(window)
   • find the window and make it active. Active means that all test commands are executed in this window. If the window is not found throws an error.
4. closeWindow(window?)
   • the window parameter is optional. If window is not given the current active window will be closed
   • if we try to close the parent from the child the error message should appear
   • if window is not found throws error
     Example:

test('test', async t => {
    const parent  = await t.getCurrentWindow();
    const window1 = await t.openWindow('/join-chat?id=1');
    const window2 = await t.openWindow('/join-chat?id=1');

    await t
        .switchToWindow(window1)
        .typeText('input', 'hello')
        .switchToWindow(window2)
        .typeText('input', 'world')
        .switchToWindow(parent)
        .closeWindow(window1)
        .closeWindow(window2)
});

5. switchToParentWindow()
   • make parent window of the current window active. If parent window is not found throws error.
6. switchToRecentWindow()
   • without history. Only back to previous. If recent window was closed for some reason throws error.
7. switchToWindow(predicate: ({ url: Object, title: string }))
   • find window by predicate and switches to it. If window is not found throws error.

switchToWindow(w => w.title === 'test title');
switchToWindow(w => w.location.href === 'http://example.com')
switchToWindow(w => w.title === 'Title' && w.location.host === 'example.com')

    w.title === 'Title'
    w.location.href === fullUrl &&
    w.location.protocol === 'http:' &&
    w.location.host === 'localhost:3000' &&
    w.location.port === '3000' &&
    w.location.query === '/fixtures/run-options/allow-multiple-windows/pages/api/parent.html';

8. We can use described methods only if the allow-multiple-windows option is enabled. In other case, the error You should activate multi window mode (enable the "allow-multiple-windows" run option) to use the "<methodName>" method appears