fix(VTextField): detect activeElement from shadowRoot

[matthieusieben]

Description

When an input element is inside a shadowDom, and is active (focused), the document.activeElement will not reference the input element but rather its shadowRoot. In order to properly detect what the activeElement is, we must use the proper root node for that element, as explained on MDN.

This PR:

• Considers whether an element is attached to the DOM when computing tryAutofocus, and during the processing of onFocus event.
• Correctly detects the focused element event if the component is inside a Shadow DOM (solving part of [Bug Report] Weird behaviour inside shadow DOM #7622)
• Improves the quality of tests by enforcing focus/activeElement dependent tests to attach the mounted components to the DOM (read below for motivation)
• Uses a shim to support IE11 (which does not have Shadow DOM)

Motivation and Context

This is part of the solution for #7622 ("Weird behaviour inside shadow DOM"). If fixes focus related issues when using Vuetify inside a shadow DOM.

It also improves the code base by attaching the element to the DOM for tests related to focus. This is important because Browsers (as well as jsdom used in tests) have a different behavior when trying to focus an element that is attached, and not (namely, when you focus an element that is not attached, is will not become the documet.activeElement).

How Has This Been Tested?

unit

Markup:

// Paste your FULL Playground.vue here

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)