added method to detect if a yaml fragment is a sequence

[mizrael]

Added a new CheckIsSequence method to detect if a yaml fragment is a sequence. It internally creates a new YamlDotNet deserializer that uses the custom type converter YamlSequenceTesterConverter.
This converter will simply check if the YAML fragment starts with the SequenceStart event and ends with SequenceEnd.

The following benchmark shows the results of a few deserialization methods:

• Direct deserialization to IEnumerable<object> using a basic YamlDotNet deserializer
• Direct deserialization to object[] using a basic YamlDotNet deserializer
• Direct deserialization to Dictionary<string,object>[] using a basic YamlDotNet deserializer
• the newly introduced CheckIsSequence method
• deserialization using the v2 deserializer
• deserialization using the v3 deserializer
  

results from CheckIsSequence are not so different from Direct deserialization to IEnumerable<object>. We might decide to remove the custom YamlSequenceTesterConverter in the future and rely directly on the base library instead.

[joem-msft]

please simplify this unit tests so it doesn't require external files for the baseline cases.
use targeted input strings for testing, don't use existing large files as these can change due to their purpose etc.

[joem-msft]

we should not need to use exceptions as control-flow.

Another design option would be to deseralize into a custom type, register a converter for that type, and in the converter populate a property which has the 'result' of the detection.

e..g

private record SequenceDetectorResult
{
  public required bool IsSequence { get; init; }
}

[joem-msft]

when using the SequenceDetectorResult pattern I mentioned above, this converter could just have this SequenceStart check, completely skip the contents, and then return the result value.

the benefit here is that there wouldn't be an exception thrown for code flow control.

[joem-msft]

I think this is a simple imlpementation that doesn't require calling the Deserializer infrastructure:

    public static bool DetectIsSequenceAtRoot(string yaml)
    {
        using var reader = new StringReader(yaml);
        return DetectIsSequenceAtRoot(reader);
    }

    public static bool DetectIsSequenceAtRoot(StringReader reader)
    {
        var parser = new Parser(reader);

        // Need to consume the StreamStart and DocumentStart events to get to the root sequence
        parser.TryConsume<StreamStart>(out _);
        parser.TryConsume<DocumentStart>(out _);

        return parser.TryConsume<SequenceStart>(out _);
    }