Separate addition of default and non-default value comparers.

[midgleyc]

Currently, replacing only the default value comparer while keeping all others the same involves rather a lot of code:

var existingComparers = Service.Instance.ValueComparers.ToList();
foreach (var existingComparer in existingComparers)
{
	Service.Instance.UnregisterValueComparer(existingComparer);
}

Service.Instance.RegisterValueComparer(new NullConversionDefaultValueComparer());
foreach (var existingComparer in existingComparers.Where(x => !(x is DefaultValueComparer)))
{
	Service.Instance.RegisterValueComparer(existingComparer);
}

This change separates the handling of default and non-default value comparers, and stops it splitting on the type.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added an entry to the changelog

[SabotageAndi]

@midgleyc Thanks for the PR.
Please add an entry to the changelog and I will merge the PR. Thanks!

[midgleyc]

I've added the entry, cheers.