J2N.Text.StringBuffer: Fixed bugs and added new APIs

[NightOwl888]

New APIs:

namespace J2N.Text
{
    public class StringBuffer
    {
        public void CopyTo(int sourceIndex, Span<char> destination, int count); // .NET Framework 4.5+
        public StringBuffer Insert(int index, ReadOnlySpan<char> value); // New legacy support for .NET Framework 4.5+
        public StringBuffer InsertCodePoint(int index, int codePoint);
    }
}

Bugs Fixed:

• Directory.Build.Targets: Removed FEATURE_CHARARRAYPOINTERS, which is a duplicate of FEATURE_STRINGBUILDER_APPEND_CHARPTR
• J2N.Text.StringBuffer::Append(StringBuffer, int, int): Fixed implementation to correctly pass in StringBuffer.builder to the Append call

Performance Optimizations

• Directory.Build.targets: Added FEATURE_STRINGBUILDER_APPEND_STRINGBUILDER to optimize for when this overload is available in .NET.