fix: move setAttribute calls for proxy elements to connectedCallback

[chrisdholt]

Description

This PR fixes an issue where attributes were defined too early for the custom element, specifically as it relates to form associated elements. This work moves the work to set proxy attributes to the connected callback.

Closes #3456

Motivation & context

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.

[janechu]

I have created a branch with createElement and applied your changes - users/janechu/custom-element-test, still getting an error.

[janechu]

By setting appearance as undefined in the button this appears to fix the problem, I think this indicates these attr default settings are happening during the ctor phase, causing an issue. See 726db0c

[chrisdholt]

By setting appearance as undefined in the button this appears to fix the problem, I think this indicates these attr default settings are happening during the ctor phase, causing an issue. See 726db0c

Yes, as I was building your branch I realized that I had started working on that, but it was on your branch. I jumped back to apply some changes and spaced. I think we'll need to set the default appearance differently.

[chrisdholt]

@janechu pushed an update. I don't get an error thrown here when testing on your branch. @EisenbergEffect can you take one more look at this when available? One slight change made since you signed off.

[janechu]

I've built and verified, looks good!