fix(fast-element): preserve observable values from before upgrade

[EisenbergEffect]

Description

Motivation & context

When observable property values are set through the template engine, this happens before the elements have been upgraded. As a result, the property values shadow the observable property getter/setter, preventing observation. Additionally, if the element initialized the properties to a starting value, the initializers will overwrite the values provided by the template engine.

This PR captures values set by the template engine, deletes the shadowing properties, and then re-applies the bound values during the connect lifecycle.

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

As part of the fix, a new API was added to enable getting a list of all observed properties for an object. The naming observedProperties was chosen to mirror the web components naming of observedAttributes.

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.