Add frequencies function

[phanan]

Summary

• Adds frequencies($collection, ?callable $callback), which returns an array mapping each distinct value to the number of times it appears in the collection
• An optional callback can be provided to count by a derived key (receives $element, $index, $collection)
• Inspired by Clojure's frequencies and Ruby's Enumerable#tally

use function Functional\frequencies;

$data = ['apple', 'banana', 'apple', 'cherry', 'banana', 'apple'];
frequencies($data); // ['apple' => 3, 'banana' => 2, 'cherry' => 1]

// With callback
$words = ['hi', 'hello', 'hey', 'goodbye', 'go'];
frequencies($words, fn ($w) => strlen($w)); // [2 => 2, 5 => 2, 3 => 1, 7 => 1]

Test plan

• Unit tests covering arrays, iterators, callbacks, empty collections, numeric values, invalid keys, and invalid arguments (7 tests, 19 assertions)