String array visitor

[airbreather]

Probably depends on #7.

Desired usage:

public sealed class MyVisitor : CsvUTF8StringArrayReaderVisitorBase
{
    public MyVisitor(int maxFieldLength, int maxFieldCount, DecoderFallback decoderFallback)
        : base(maxFieldLength, maxFieldCount, decoderFallback)
    {
    }

    public override void VisitRecord(ReadOnlySpan<string> fields)
    {
        // do stuff with all the fields for this record
    }
}

[airbreather]

I think we can skip the Encoding like #5 did... updating.