Add Decimal32, Decimal64, Decimal128

[RaymondHuy]

Resolve #81376

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[huoyaoyuan]

If Int128 is not optimized, this can be optimally stored as a ReadOnlySpan of Int32/Int64 like Number.BigInteger.Pow10BigNumTable

[huoyaoyuan]

+0 and -0 should have the same hash code. The easiest way can be stripping the sign bit.

I also remember that there can be same values with different representations.

runtime/src/libraries/System.Private.CoreLib/src/System/Numerics/TotalOrderIeee754Comparer.cs

Lines 125 to 132 in 9b57a26

	else
	{
	// Equivalant values are compared by their exponent parts,
	// and the value with smaller exponent is considered closer to zero.
	// This only applies to IEEE 754 decimals. Consider to add support if decimals are added into .NET.
	return 0;
	}

[RaymondHuy]

@huoyaoyuan Thanks for your review. However this PR targets only the first phase as following: #81376 (comment) so I haven't added NegativeZero and PositiveZero. I will update GetHashCode method in the next phase.

[tannergooding]

This is on my radar to take a look at; just noting it might be a bit delayed due to other priorities for the .NET 9 release.

CC. @jeffhandley

[RaymondHuy]

Hi @tannergooding I have resolved almost your comments (some places need your resolve confirmation). For the constructor API case, I don't know what I should do with it because I think you are going to raise it in the API Proposal to remove this constructor.

[RaymondHuy]

Hi @tannergooding this PR is ready to review, I have separated the processing flow for Parse and Constructor(significand, exponent) also added the comments so that it's simpler than the previous one you reviewed.