J2N.Character: Added ReadOnlySpan<char> overloads for working with codepoints

[NightOwl888]

Added ReadOnlySpan overloads for CodePointAt, CodePointBefore, CodePointCount, and OffsetByCodePoints + tests

New APIs

namespace J2N
{
    public static class Character
    {
        public static int CodePointAt(this ReadOnlySpan<char> seq, int index)
        public static int CodePointBefore(this ReadOnlySpan<char> seq, int index)
        public static int CodePointCount(this ReadOnlySpan<char> seq)
        public static int OffsetByCodePoints(this ReadOnlySpan<char> seq, int index, int codePointOffset)
    }
}