refactor(forms): Move FormControl to an overridden constructor.

[dylhunn]

This implementation change was originally proposed as part of Typed Forms, and will have major consequences for that project as described in the design doc. In summary, this change causes the constructor for FormControl to be decoupled from its implementation. This will allow us to provide multiple constructor signatures in the future, and even to return FormControl<T> where T takes different values depending on the constructor signature selected. (For example, in the simple case, new FormControl<string>('foo') will return FormControl<string|null>.)

Accordingly, a new exported symbol is created to provide various constructor overrides, with documentation examples for how to use the different constructors. (Currently there are only two, but there will be more after Typed Forms/default values.)

Submitting this change separately is desirable for a few reasons:

• It reduces the risk of landing Typed Forms -- the constructor override is complicated, and is fundamentally separate from the core types.
• It enables us to submit a follow-up change for default values before Typed Forms lands, thus separating the only runtime behavior change from all the types-only changes.

This change should have no visible impact on users of FormControl.

See the Typed Forms design doc here: https://docs.google.com/document/d/1cWuBE-oo5WLtwkLFxbNTiaVQGNk8ipgbekZcKBeyxxo.

#13721

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

• Bugfix
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[dylhunn]

Presubmit is clean (barring one flake). We'll need a TGP before submitting as well, just to be safe.

[sonukapoor]

LGTM

[AndrewKushnir]

Looks good, just left a couple comments 👍

[mary-poppins]

You can preview 48883c0 at https://pr44316-48883c0.ngbuilds.io/.
You can preview 789c9aa at https://pr44316-789c9aa.ngbuilds.io/.

[dylhunn]

@AndrewKushnir I have made the requested changes.

[mary-poppins]

You can preview d5015a1 at https://pr44316-d5015a1.ngbuilds.io/.

[mary-poppins]

You can preview 953989c at https://pr44316-953989c.ngbuilds.io/.

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