BUG: incorrect parsing with non-dot decimal separator

[Bykiev]

Description
When rounding the value with culture where decimal separator is comma an error has occured.

Error/Exception Message
System.FormatException: "The value could not be parsed."

в System.Numerics.BigNumber.ParseBigInteger(ReadOnlySpan`1 value, NumberStyles style, NumberFormatInfo info)
   в System.Numerics.BigNumber.ParseBigInteger(String value, NumberStyles style, NumberFormatInfo info)
   в ExtendedNumerics.BigDecimal.GetWholePart()
   в ExtendedNumerics.BigDecimal.Round(BigDecimal value, MidpointRounding mode)

Expected Behavior
The value should be correctly parsed

Actual Behavior
An error has occured: System.FormatException: "The value could not be parsed."

Steps To Reproduce

Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");
Thread.CurrentThread.CurrentCulture.NumberFormat.NumberDecimalSeparator = ",";

var res = new BigDecimal(17.56);
var e = BigDecimal.Round(res, MidpointRounding.AwayFromZero);

[Bykiev]

I'm working on this, will create PR soon