Add Parse methods for Span

[Joy-less]

• Adds BigDecimal.Parse(ReadOnlySpan<char>), BigDecimal.Parse(ReadOnlySpan<char>, IFormatProvider?), BigDecimal.TryParse(ReadOnlySpan<char>, out BigDecimal), BigDecimal.TryParse(ReadOnlySpan<char>, out BigDecimal, IFormatProvider?) for .NET 8.0 and greater
• Optimises BigDecimal.Parse(string, IFormatProvider) / BigDecimal.Parse(string)
• Fixes a bug where e (exponent) can't be lowercase in Parse(input)
• Many changes listed here are from #67.