feat(forms): Add FormBuilder.nonNullable.

[dylhunn]

With typed forms, all FormControls are nullable by default, because they can be reset to null. This behavior is possible to change by passing the option initialValueIsDefault: true. However, in a large form, this is extremely cumbersome, as the option must be repeated over and over. Additionally, it is not possible to take full advantage of FormBuilder, since FormBuilder.group and FormBuilder.array will produce nullable controls.

This PR introduces a new accessor FormBuilder.nonNullable, which produces non-nullable controls. Specifically, any call to .control will produce controls with {initialValueIsDefault: true}, and calls to .array or .group that implicitly build inner controls will have the same effect.

let nfb = new FormBuilder().nonNullable;
let name = nfb.group({who: 'Alex'}); // FormGroup<{who: FormControl<string>}>
name.reset();
console.log(name); // {who: 'Alex'}

Issue: #13721

[dylhunn]

After discussions with Andrew K and Alex, here are the salient points:

• nonNull -> nonNullable
• Provide a field FormBuilder.nonNullable instead of a separate class
• Implementation should be directly on FormBuilder instead of in a subclass, cleaning up some of the messiness

And then in a follow-up change:

• new FormControl('foo', {initialValueIsDefault: true}); -> new FormControl('foo', {nonNullable: true});

I'll make these fixes this afternoon.

[dylhunn]

This has been updated with the changes above.

[dylhunn]

@alxhub Thanks, I've addressed your comments!

[atscott]

reviewed-for: public-api

[alxhub]

Reviewed-for: public-api

[alxhub]

Should we perhaps assert here that initialValueIsDefault is not explicitly set to false?

[AndrewKushnir]

Reviewed-for: public-api

[dylhunn]

merge-assistance: a small nit was fixed, approvals above.

[dylhunn]

This PR was merged into the repository by commit 2dbdebc.

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