Checking for 'func' command works on MacOS too

[eashi]

This PR changes the code that checks for the func command in PATH so that it works on both Windows and MacOS.

The current code only works on Windows:

1. The path separator is a semicolon (doesn't work on Mac). This PR changes the code so that it uses the Path.PathSeparator rather than hard-coding a semicolon or a colon. For more information check: https://learn.microsoft.com/en-us/dotnet/api/system.io.path.pathseparator?view=net-7.0
2. The check assumes that the command file func has the extension suffix .exe. This is true on Windows, but not on MacOS. This PR searches for both cases to support both Windows and MacOS.
3. The Environment.GetEnvironmentVariable("PATH", EnvironmentVariableTarget.Machine) returns null because the option passed EnvironmentVariableTarget.Machine fails on MacOS. Removing the option fixes it and shouldn't cause any unexpected behaviours. For more infomration check https://learn.microsoft.com/en-us/dotnet/api/system.environment.getenvironmentvariable?view=net-7.0

[mark-abrams]

Thanks for the contribution. I've only used the framework on a Windows OS, so it is good to see that someone is using on a MacOS. Have you run into any other problems or issues when using the testing framework on MacOS?

[eashi]

Thanks for accepting the PR. I haven't came across any other issues, but will keep you posted. Cheers.

[mark-abrams]

Hi @eashi , I have uploaded version 1.6.0 of LogicAppUnit to Nuget. This includes your changes in PR #14 and also PR #15 for support for the MacOS platform. Thank you for your contribution.

[eashi]

This is great news, thanks Mark for the great project and active maintenance.