To build the playground yourself, simply do:
./gradlew :playground:shadowJar
That will make the file playground/build/libs/playground-0.0.1-all.jar
To run it, give this command:
java -cp playground/build/libs/playground-0.0.1-all.jar no.priv.garshol.jslt.playground.PlaygroundServer 9999 &
Then go to http://localhost:9999/jslt and you'll have the playground
right there.