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

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/ref/System.Runtime.cs:1

• These new numeric types expose ToString(string?, IFormatProvider?) overloads but do not implement IFormattable. As a result, composite formatting / interpolated string format specifiers (e.g., string.Format(\"{0:N}\", value) or $\"{value:N}\") may ignore the format string and fall back to parameterless ToString(). Consider adding System.IFormattable to Decimal32/Decimal64/Decimal128 (and updating the CoreLib implementations accordingly); optionally also implement ISpanFormattable to match other numerics.

[RaymondHuy]

Hi @tannergooding I have resolved all Copilot comments. You can re-review it when you have time ;)