Improve VerseRef.TrySetVerseUnicode by handling non-decimal numerals and surrogate pair numerals.

[SamDelaney]

This change would allow applications to more easily and broadly use vernacular versification.

The methods char.IsDigit and char.GetNumericValue (used in VerseRef.TryGetVerseNum) only handle base-10, non-surrogate numerals (Complete list here). A broader alternative to these methods would have to be found or made.

The value calculation in TrySetVerseUnicode also currently assumes a decimal base:
vNum = (short)(vNum * 10 + (romanOnly ? ch - '0' : char.GetNumericValue(ch)));
This would have to be changed.

See discussion on #999 for additional context and (ignored) test cases.

[ermshiperete]

.NET 5 (and .NET Core 3+) has Rune.IsDigit and Rune.GetNumericValue methods that can deal with surrogate pairs.

Alternatively this could be implemented with Char.IsDigit/Char.IsNumber and Char.IsSurrogate to identify the first part of the string that represents the number and Int32.TryParse to convert the first part of the string to a number.