fix: ensure StyleElementCustomPropertyManager works with disconnected elements

[nicholasrice]

Description

This change adds behaviors that allows using the StyleElementCustomPropertyManager with a client element that is not connected.

fixes #4109

Motivation & context

Prior to this change, calling document.createElement('fast-design-system-provider') in FireFox would cause the StyleElementCustomPropertyManager to throw because it would attempt to access a property of null. The property is null when the <style> element provided to the manager is not connected to the DOM, which happens during programmatic creation of the fast-design-system-provider because the DSP itself is disconnected from the DOM.

This change checks if the problematic property is null, and if so, it attaches a subscriber to the isConnected property of the element's controller. When the subscriber invokes, only then is the customPropertyTarget accessed and assigned.

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.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

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.

[EisenbergEffect]

Just one thing I'm not quite sure about. If we can resolve that, I can approve.

[marjonlynch]

This definitely fixes the issue in firefox, smoked