[API Proposal]: MemoryExtensions.CommonSuffixLength<T>

### Background and motivation

I've run into wanting this a few times over the years - most recently, it was for subtracting two BigInt's (my own custom ones, which done similarly to the normal BigInteger, but slightly differently to the BCL ones) - but this is not the only case I've run into wanting this for (I could try to find other cases if wanted, but might take me a while to actually find the relevant code, as I don't remember off the top of my head where it is or what it was for).

This to me doesn't seem any different to having both `IndexOf` and `LastIndexOf`, despite `IndexOf` being used much more overall.

### API Proposal

```csharp
namespace System
{
public static class MemoryExtensions
{
+ public static int CommonSuffixLength(this ReadOnlySpan span, ReadOnlySpan other);
+ public static int CommonSuffixLength(this ReadOnlySpan span, ReadOnlySpan other, IEqualityComparer? comparer = null);
}
}
```

### API Usage

```csharp
var bits1 = ...;
var bits2 = ...;

if (bits1.Length == bits2.Length)
{
var commonSuffixLength = bits1.CommonSuffixLength(bits2);
bits1 = bits1[..^commonSuffixLength];
bits2 = bits2[..^commonSuffixLength];
}

...
```

### Alternative Designs

We could also add the `this Span` ones for consistency with `CommonPrefixLength`, but idk if that's necessary with first class span these days, although it probably helps with languages like VB.NET using it.

If https://github.com/dotnet/runtime/issues/126756 gets approved, then we probably want a `public static int CommonSuffixLength(this ReadOnlySpan other, StringComparison comparisonType);` also to match it.

### Risks

_No response_