Enabling Parrot Parser for Groovy Eclipse

Since spotless depends on groovy-eclipse for groovy parsing/formatting, and since groovy-eclipse doesn't enable the parrot parser by default, Spotless fails when configured for groovy code that utilizes Groovy 3+ syntax.

After experiencing some pain with spotless + groovy, I stumbled upon #246 which led me to dig into groovy-eclipse. Turns out groovy-eclipse leaves the new antlr4 based Parrot Parser [disabled by default](https://github.com/groovy/groovy-eclipse/wiki/4.2.0-Release-Notes#support-for-new-versions-of-groovy), which means any newer Groovy 3 syntax that contains, for example, Java lambdas, blows up.

I can manually add `-Dgroovy.antlr4=true` to my Gradle execution and that gets it working for me locally, but there should be some way to configure this right within build.gradle? I'm trying to find a solution that is more amenable to a team environment.

In the past I've modified JavaExec tasks and the like to include forked JVM options. But there seems to be no way to do that for SpotlessTask.

Am I missing something obvious here?