Fix Previewer GitHub Release download for macOS

It was not possible to find any compatible framework version
The framework 'Microsoft.WindowsDesktop.App', version '3.1.0' was not found.
  - No frameworks were found.

You can resolve the problem by installing the specified framework and/or SDK.

The specified framework can be found at:
  - https://aka.ms/dotnet-core-applaunch?framework=Microsoft.WindowsDesktop.App&framework_version=3.1.0&arch=x64&rid=osx.10.15-x64

WinForms is only available on windows (note that .NET 6 will have a cross-platform GUI)

[Auburn]

You can try running this command to publish, it should bundle it as a .app

dotnet publish .\FastNoiseLiteGUI.csproj -c Release -r osx-x64 --self-contained

[Zorgatone]

@Auburn is the new binary without an extension?

I tried this two ways and didn’t work:

 ~/Downloads  dotnet ./FastNoiseLiteGUI-OSX64                                                                                                         ✔  6611  13:02:52
Could not execute because the specified command or file was not found.
Possible reasons for this include:
  * You misspelled a built-in dotnet command.
  * You intended to execute a .NET program, but dotnet-./FastNoiseLiteGUI-OSX64 does not exist.
  * You intended to run a global tool, but a dotnet-prefixed executable with this name could not be found on the PATH.
 ~/Downloads  ./FastNoiseLiteGUI-OSX64                                                                                                              1 ↵  6612  13:03:01
zsh: permission denied: ./FastNoiseLiteGUI-OSX64
 ~/Downloads  chmod +x FastNoiseLiteGUI-OSX64                                                                                                     126 ↵  6613  13:03:06
 ~/Downloads  ./FastNoiseLiteGUI-OSX64                                                                                                                ✔  6614  13:03:10
[1]    4972 killed     ./FastNoiseLiteGUI-OSX64
 ~/Downloads  ./FastNoiseLiteGUI-OSX64                                                                                                     SIGKILL(9) ↵  6615  13:03:31
Unhandled exception. System.InvalidOperationException: Could not detect platform. Are you missing a platform assembly?
 ---> System.InvalidOperationException: Platform type 'Eto.Mac.Platform, Eto.Mac64' was loaded but is not valid in the current context.  E.g. Mac platforms require to be in an .app bundle to run
   --- End of inner exception stack trace ---
   at Eto.Platform.get_Detect()
   at Eto.Forms.Application..ctor()
   at Program.Main(String[] args)
[1]    5889 abort      ./FastNoiseLiteGUI-OSX64