New: Linux Support

This change help Antlr to run on Linux.

To test manually add in your csproj the g4 files just like that:

```xml
<Antlr4 Include="G4_FILE">
      <Generator>MSBuild:Compile</Generator>
      <CustomToolNamespace>YOUR_NAMESPACE</CustomToolNamespace>
      <Visitor>True</Visitor> <!-- True or False -->
      <Listener>True</Listener> <!-- True or False -->
      <TargetLanguage>CSharp</TargetLanguage> <!-- Language -->
</Antlr4>
```

On Linux check if your JAVA_HOME is correctly set up.
I'm a little bit uncomfortable about this change, but I understand the problem it solves for you. I want to think about this for a bit and see if there's a better platform-agnostic solution but I may end up going with this in the end.