fix(typings): correctly unpack values of nested form group arrays

[ntziolis]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/reactive-forms/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

Minor fix for existing tying of ValuesOf for nested FormArray<FormGroup>.

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Assuming the following FormType:

export interface FormType {
  items: {
    tags: FormControl<string[]>
  }[]
}

When using ValuesOf<ControlsOf<FormType>> the tags has the type FormControl<string[]> instead of string[].

Issue Number: N/A

What is the new behavior?

The typing now correctly unpacks the values of FormGroup nested in FormArrays.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Users might encounter a breaking change if they have the above scenario AND rely on the current incorrect typing. Fixing resulting errors is likely low effort

Other information

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