Add a @byContent selector for reparenting child content

Description

This is the implementation of a @byContent decorator that would allow the capturing of arbitrary DOM content that the web component user has placed into a wrapped web component.

Motivation and Context

Adding this feature would allow us to turn a React container component into a web component. So, we could write containers such as popovers and dialogs in React and wrap them as web components. The user of the web components could pass in arbitrary content to be hosted in the container.

How Has This Been Tested?

I have developed a basic container control using this code and I have added a new test to the test suite to test it.

Screenshots (if appropriate):

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[x ] I have signed the Adobe Open Source CLA.
[x ] My code follows the code style of this project.
[x ] My change requires a change to the documentation.
[x ] I have updated the documentation accordingly.
[ x] I have read the CONTRIBUTING document.
[x ] I have added tests to cover my changes.
[x ] All new and existing tests passed.

adobe / react-webcomponent