feat: convert FormAssociated to a constructable function

[radium-v]

Description

Converts FormAssociated to be a "constructable" function call that returns a mixin class.

Motivation & context

Allows for easier control when working with mixin classes and FormAssociated.

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
This modifies all FormAssociated-based components:

• Button

• Checkbox

• Radio

• Slider

• Text Area

• Text Field

• 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.

[radium-v]

@nicholasrice there are still two tests that are breaking for me which were added recently, can you take a look and see if there's a quick fix?

[EisenbergEffect]

I like this change. I made one request to move an interface. If we can make that change and get the tests all passing, I think we're good.

[nicholasrice]

I think the FormAssociated test assertion can be changed to expect empty string. At this point I don't recall why that test is there, but if you check the value property of an <input> prior to connection it is empty string, so I think we should match that behavior.

[nicholasrice]

Thanks @radium-v - this is awesome.

[chrisdholt]

That's a lot of updates :)

Thanks @radium-v!