add @main entrypoint support for Scala 3

mainargs 0.2.3 added support for scala3 🎉
com-lihaoyi/mainargs#18

The first attempt to upgrade and enable mainargs was
com-lihaoyi#1298, but the build
changes were better handled by com-lihaoyi#1301

So the only thing left is reenabling mainargs for Scala 3.

amm/src/main/scala/ammonite/main/Scripts.scala

@@ -17,12 +17,7 @@ object Scripts {
                 scriptArgs: Seq[String] = Nil) = {
     interp.watch(path)
     val (pkg, wrapper) = Util.pathToPackageWrapper(Seq(), path relativeTo wd)
-
-
-    val genRoutesCode =
-      // Entrypoints are not supported in Scala 3 for now (its macros are Scala 2-only)
-      if (interp.compilerBuilder.scalaVersion.startsWith("3.")) "null"
-      else "mainargs.ParserForMethods[$routesOuter.type]($routesOuter)"
+    val genRoutesCode = "mainargs.ParserForMethods[$routesOuter.type]($routesOuter)"
 
     for{
       scriptTxt <- try Res.Success(Util.normalizeNewlines(os.read(path))) catch{