feat: non-uniform custom comparer support across assertion types

[thomhurst]

Description

Custom comparer support varies significantly across assertion types:

• CollectionContainsAssertion accepts IEqualityComparer<TItem>? in constructor
• IsEquivalentToAssertion has .Using(Func<...>) method
• DictionaryContainsKeyAssertion has both .Using(IEqualityComparer<TKey>) and .Using(Func<...>)
• Some assertions accept comparer in constructor, others via fluent method

Suggested Fix

Standardize the API:

1. Always use .Using() for fluent API (consistent method name)
2. Document semantics clearly (does it replace or augment default comparer?)
3. Support both IEqualityComparer<T> and Func<T, T, bool> overloads uniformly