v5.1.3

[alexrp]

• Changed UnicodeEnvironment to also control the _UNICODE macro (in addition to UNICODE).
• Changed TargetFramework property from netstandard1.0 to netstandard2.0 to address a NETSDK1215 warning in .NET 9.
  • This should not cause any functional changes as the target framework is irrelevant for projects using the Zig SDK.