Cross-publish example and launcher artifacts to maven central (#4372)

This is a step to moving them off of Github to Maven Central, where we
have stronger guarantees (e.g. immutability) and a narrower service
dependency list (Mill already depends on maven central anyway, and this
removes the dependency on Github). With 0.13.0 I expect to remove these
artifacts from Github Releases entirely



We still have some Github release download dependencies, e.g. some JVM
downloads come from Github, but at least for things that are within our
control it's better for them to be on maven central

dist/package.mill

@@ -329,6 +329,15 @@ object `package` extends RootModule with InstallModule {
     longUrl
   }
 
+  def publishArtifacts = Task {
+    super.publishArtifacts()
+      .copy(payload =
+        super.publishArtifacts().payload ++
+          exampleZips().map(z => (z, z.path.last)) ++
+          Seq((bootstrapLauncher(), "mill"), (bootstrapLauncherBat(), "mill.bat"))
+      )
+  }
+
   object native extends mill.scalalib.NativeImageModule with InstallModule {
     def artifactOsSuffix = T {
       val osName = System.getProperty("os.name").toLowerCase