Add flag to show prelude (#1032)

Solves #915 and allows some highlighting on `effekt-api`.

Author: marvinborner

effekt/jvm/src/main/scala/effekt/EffektConfig.scala

@@ -135,6 +135,15 @@ class EffektConfig(args: Seq[String]) extends REPLConfig(args.takeWhile(_ != "--
   // ---------
   private val debugging = group("Compiler Development")
 
+  val showPrelude: ScallopOption[Boolean] = toggle(
+    "show-prelude",
+    descrYes = "Print modules to be automatically imported in every file",
+    default = Some(false),
+    noshort = true,
+    prefix = "no-",
+    group = debugging
+  )
+
   val showIR: ScallopOption[Option[Stage]] = choice(
     choices = List("none", "core", "machine", "target"),
     name = "ir-show",

effekt/jvm/src/main/scala/effekt/Main.scala

@@ -21,7 +21,9 @@ object Main {
         return
     }
 
-    if (config.server()) {
+    if (config.showPrelude()) {
+      println(config.prelude().mkString(","))
+    } else if (config.server()) {
       val serverConfig = ServerConfig(
         debug = config.debug(),
         debugPort = config.debugPort()