Convert `ExitCodes` constants class to an `enum`

[Evangelink]

Priority: Low
Estimated Effort: Small

src/Platform/Microsoft.Testing.Platform/Helpers/ExitCodes.cs:12 has a TODO to convert to an enum and rename to ExitCode (singular). This improves type safety, discoverability, and is consistent with .NET conventions. This would be a breaking change to the public API surface and should be planned for the next major version.

[Youssef1313]

My original thinking that this is a breaking change might not be correct, as the exit codes are declared as const and not static readonly. But please double check.