refactor!: decouple purity, validity, and submission status

[felangel]

Description

This pull request attempts to decouple purity from validity (closes #45) as well as to decouple submission status from validity/purity (closes #46).

Changes

1. FormzStatus renamed to FormzSubmissionStatus:

/// Enum representing the submission status of a form.
enum FormzSubmissionStatus {
  /// The form is in the process of being submitted.
  inProgress,

  /// The form has been submitted successfully.
  success,

  /// The form submission failed.
  failure,

  /// The form submission has been canceled.
  canceled
}

2. FormzInput class no longer exposes a status (FormzInputStatus). Instead there are isValid and isNotValid getters:

class NameInput extends FormzInput<String, NameInputError> {
  const NameInput.pure() : super.pure('');
  const NameInput.dirty({String value = ''}) : super.dirty(value);

  @override
  NameInputError? validator(String value) {
    return value.isNotEmpty == true ? null : NameInputError.empty;
  }
}

void main() {
  const name = NameInput.pure();
  print(name.isValid); // false
  print(name.isNotValid); // true

  const joe = NameInput.dirty(value: 'joe');
  print(joe.isValid); // true
  print(joe.isNotValid); // false
}

3. FormzInput has a displayError getter which returns an error to display if the input is not valid and has been modified by the user (closes Determining when to show meaningful errors #44)

void main() {
  const name = NameInput.pure();
  print(name.displayError); // null

  const invalid = NameInput.dirty(value: '');
  print(name.displayError); // NameInputError.empty
}

4. Renamed pure to isPure for consistency

5. Use very_good_analysis

[codecov-commenter]

Codecov Report

Merging #48 (b3fb409) into main (26a22df) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #48   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           31        27    -4     
=========================================
- Hits            31        27    -4     

Impacted Files	Coverage Δ
lib/formz.dart	100.00% <100.00%> (ø)

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[jorgecoca]

I like the simplification! 💯

[definitelyokay]

thank goodness ❤️

[MJ12358]

Love the changes! Thanks for this!
Is there any chance on making this a release and publishing to pub.dev?

[felangel]

Love the changes! Thanks for this!
Is there any chance on making this a release and publishing to pub.dev?

Thanks! Yup will try to get a release out shortly.