Add Math.Tau, MathF.Tau

[john-h-k]

Added Math.Tau and MathF.Tau constants. Tests for Math/F.E and Math/F.PI were also introduced to check they are the expected value for consistency 😄

Fixes #24678

[Dotnet-GitSync-Bot]

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, to 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.

[john-h-k]

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, to 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.

Neither E nor PI have triple slash docs so for consistency I stayed that way, but I see that Tau is a less well-known constant so they may be useful

[john-h-k]

cc @tannergooding

[tannergooding]

Just noting for anyone that comes looking, the value we put in the src is the actual value of PI to at least 17 significant digits + 1 rounding digit (or to at least 9 significant digits + 1 rounding digit for float).

The value here is the actual double or float value (which may be different due to rounding as the actual value may not be exactly representable) formatted as a string with at least enough precision to correctly roundtrip. Hence why the ref float versions differ slightly from the "true digits" of PI or TAU

REF FLT: 3.1415927
SRC FLT: 3.1415926 5
REF DBL: 3.1415926 53589793
SRC DBL: 3.1415926 53589793 23846
TRU PI:  3.1415926 53589793 23846 2643383279502884197169399375105820974...

REF FLT: 6.2831855
SRC FLT: 6.2831853 07
REF DBL: 6.2831853 07179586
SRC DBL: 6.2831853 07179586 476925
TRU TAU: 6.2831853 07179586 476925 286766559005768394338798750211641949...

[aaronfranke]

Should this be placed on the 5.0 milestone, since I assume it will be available in .NET 5?