Make --bsp Mill process launch from MillBspMain class (#5896)

This should make the `MillBspMain` processes easier to distinguish from
the command line from the `MillDaemonMain` or `MillNoDaemonMain`
processes launched by users when you run `jps`.

Tested manually via `./mill dist.installLocalCache` and using the new
version in a Mill project I load into Intellij. `jps` then shows the
`MillBspMain` clearly separate from the other user-created Mill
processes:

```console
$ jps
4930 MillNoDaemonMain
1717 
7063 MillLauncherMain
7080 Jps
5835 MillDaemonMain
749 Main
7070 MillBspMain
4927 NailgunRunner
```

Author: lihaoyi

runner/daemon/src/mill/daemon/MillBspMain.scala

@@ -0,0 +1,5 @@
+package mill.daemon
+
+object MillBspMain {
+  def main(args0: Array[String]): Unit = MillNoDaemonMain.main(args0)
+}

runner/launcher/src/mill/launcher/MillLauncherMain.java

@@ -88,8 +88,11 @@ public static void main(String[] args) throws Exception {
         });
 
     if (runNoDaemon) {
+      String mainClass = bspMode ? "mill.daemon.MillBspMain" : "mill.daemon.MillNoDaemonMain";
       // start in no-server mode
-      System.exit(MillProcessLauncher.launchMillNoDaemon(args, outMode, runnerClasspath));
+      int exitCode =
+          MillProcessLauncher.launchMillNoDaemon(args, outMode, runnerClasspath, mainClass);
+      System.exit(exitCode);
     } else {
       var logs = new java.util.ArrayList<String>();
       try {

runner/launcher/src/mill/launcher/MillProcessLauncher.java

@@ -19,7 +19,8 @@
 
 public class MillProcessLauncher {
 
-  static int launchMillNoDaemon(String[] args, OutFolderMode outMode, String[] runnerClasspath)
+  static int launchMillNoDaemon(
+      String[] args, OutFolderMode outMode, String[] runnerClasspath, String mainClass)
       throws Exception {
     final String sig = String.format("%08x", UUID.randomUUID().hashCode());
     final Path processDir =
@@ -29,7 +30,7 @@ static int launchMillNoDaemon(String[] args, OutFolderMode outMode, String[] run
     l.addAll(millLaunchJvmCommand(outMode, runnerClasspath));
     Map<String, String> propsMap = ClientUtil.getUserSetProperties();
     for (String key : propsMap.keySet()) l.add("-D" + key + "=" + propsMap.get(key));
-    l.add("mill.daemon.MillNoDaemonMain");
+    l.add(mainClass);
     l.add(processDir.toAbsolutePath().toString());
     l.add(outMode.asString());
     l.addAll(millOpts(outMode));