Detect headless JREs

Crashes caused by the missing AWT dependency come up in the support channels fairly often.
This patch detects the missing dependency and stops the server with a clear error message,
containing a link to instructions on how to install a non-headless JRE.

paper-server/src/main/java/io/papermc/paper/util/ServerEnvironment.java

@@ -20,4 +20,14 @@ public class ServerEnvironment {
     public static boolean userIsRootOrAdmin() {
         return RUNNING_AS_ROOT_OR_ADMIN;
     }
+
+    public static String awtDependencyCheck() {
+        try {
+            new java.awt.Color(0);
+        } catch (UnsatisfiedLinkError e) {
+            return e.getClass().getName() + ": " + e.getMessage();
+        }
+
+        return null;
+    }
 }