Feature Request: Add `countFrequency` method to `Iterable` in the collection package

Description

I would like to propose adding a new method countFrequency to the Iterable class in the collection package. This method would return a Map where the keys are the unique elements from the iterable, and the values are the counts of those elements.

Motivation

A built-in method to count the frequency of elements in an iterable simplifies common tasks that developers often need to implement manually. This addition will enhance the collection package's usability and convenience, providing a standard way to perform this operation.

Proposed Implementation

Add the following extension to lib/src/iterable_extensions.dart:

extension FrequencyCounter<T> on Iterable<T> {
  /// Returns a map where the keys are the unique elements of the iterable
  /// and the values are the counts of those elements.
  Map<T, int> countFrequency() {
    Map<T, int> frequencyMap = {};
    for (var item in this) {
      frequencyMap[item] = (frequencyMap[item] ?? 0) + 1;
    }
    return frequencyMap;
  }
}

Example Usage

void main() {
  List<String> items = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple'];
  var frequencyMap = items.countFrequency();

  print(frequencyMap); // Output: {apple: 3, banana: 2, orange: 1}
}

If you agree, I can open a PR with code and tests for all Iterable subclasses including Edge Cases.

dart-lang / core