Rename 'registry_tool' to 'nomulus'

This changes everything with external visibility beyond the codebase
(i.e. the name of the compiled binary and the documentation that refers
to it). It does not change a lot of things internal to the codebase,
i.e. the "RegistryTool" class didn't change its name. We can rename that
in a subsequent CL if we want to.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135022087

java/google/registry/tools/RegistryTool.java

@@ -120,6 +120,6 @@ public final class RegistryTool {
 
   public static void main(String[] args) throws Exception {
     RegistryToolEnvironment.parseFromArgs(args).setup();
-    new RegistryCli().run("registry_tool", args, COMMAND_MAP);
+    new RegistryCli().run("nomulus", args, COMMAND_MAP);
   }
 }