feat(forms): export forms utility functions: isFormArray, isFormGroup…

[fmalcher]

This commit exports existing utility functions to check for control instances: isFormControl, isFormGroup, isFormRecord, isFormArray Those are useful when implementing validators that use the specifics of one of those control types. To narrow down the type to what it actually is, we can now use the util functions in validators:

export const myArrayValidator: ValidatorFn = (control) => {
  if (!isFormArray(control)) { return null; }

  // now you can use FormArray-specific members, e.g.:
  if (control.controls.every(c => !!c.value) {
    return { myerror: true }
  } else { return null; }
}

Before, this had to be done manually:

export const myArrayValidator: ValidatorFn = (control) => {
  if (!(control instanceof FormArray)) { return null; }
  // ... 
}

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/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?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[dylhunn]

reviewed-for: fw-forms, public-api

[AndrewKushnir]

Reviewed-for: public-api

[jessicajaniuk]

This PR was merged into the repository by commit a8569e3.

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