Add Decimal32, Decimal64, Decimal128

[RaymondHuy]

Resolve #81376

[ghost]

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.

[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

[PranavSenthilnathan]

@RaymondHuy can you add test cases for the last few cases you fixed:

• CompareTo/GetHashCode of different infinities/NaN
• Parsing 0.500000000001x10^-101 (parse buffer only stores 0.50000000x10^-101 but has non-zero tail)
• The counterexample you found for why normalizedCurrentSignificand approach doesn't work for comparisons

I'm good with the PR otherwise and can approve with the added tests.

[RaymondHuy]

Hi @PranavSenthilnathan, I have added more test cases following your suggestions:

CompareTo/GetHashCode of different infinities/NaN
https://github.com/RaymondHuy/runtime/blob/a8769bbe8b284104d4985e8151be77f7adb08f3d/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Decimal32Tests.cs#L455

Parsing 0.500000000001x10^-101 (parse buffer only stores 0.50000000x10^-101 but has non-zero tail)
https://github.com/RaymondHuy/runtime/blob/a8769bbe8b284104d4985e8151be77f7adb08f3d/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Decimal32Tests.cs#L335

The counterexample you found for why normalizedCurrentSignificand approach doesn't work for comparisons
https://github.com/RaymondHuy/runtime/blob/a8769bbe8b284104d4985e8151be77f7adb08f3d/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Decimal32Tests.cs#L313

[tannergooding]

@RaymondHuy looks like there's still a couple test failures:

System.Tests.Decimal128Tests.CompareTo_Other_ReturnsExpected
System.Tests.Decimal64Tests.CompareTo_Other_ReturnsExpected

Both are comparing zero (0.000.....0 vs 0.000....1) and give stack traces are similar to:

at System.UInt64.DivRem(UInt64 left, UInt64 right)
   at System.Number.<CompareDecimalIeee754>g__InternalUnsignedCompare|3_0[TDecimal,TValue](DecodedDecimalIeee754`1 current, DecodedDecimalIeee754`1 other)
   at System.Number.CompareDecimalIeee754[TDecimal,TValue](TValue currentDecimalBits, TValue otherDecimalBits)
   at System.Numerics.Decimal64.CompareTo(Decimal64 other)
   at System.Tests.Decimal64Tests.CompareTo_Other_ReturnsExpected(Decimal64 d1, Decimal64 d2, Int32 expected)
   at InvokeStub_Decimal64Tests.CompareTo_Other_ReturnsExpected(Object, Span`1)
   at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)