Support multiple file extensions explicitly

[franzbecker]

In Xtext I can have multiple file extensions for a given Xtext language by configuring the MWE2 workflow like this:

language = StandardLanguage {
    name = "com.example.Demo"
    fileExtensions = "demo,example"
}

In the xtext-gradle-plugin this does not work out of the box. I can do something like this:

xtext {
    languages {
        demo {
            fileExtension = 'demo'
            setup = 'com.example.DemoStandaloneSetup'
        }
        demoExample {
            fileExtension = 'example'
            setup = 'com.example.DemoStandaloneSetup'
        }
    }
}

but when generating Java code it seems that I need to add generator.outlet.producesJava = true to every language.

It would be nice to have a syntax such as:

xtext {
    languages {
        demo {
            fileExtensions = 'demo,example'
            setup = 'com.example.DemoStandaloneSetup'
        }
    }
}