Assume npx (as in meteor npx ...) is a valid command.

Sometimes, before dev_bundle/bin/.meteor-commands.json has been written, a
command like `./meteor npx tsc` can fail because `npx` is not blessed in
the same way `node` and `npm` are. Since `npx` is a core tool that we want
folks to be able to use without confusion, I think it makes sense to
include it in the list of known subcommands.

tools/cli/dev-bundle-bin-helpers.js

@@ -38,7 +38,8 @@ exports.getCommand = function (name, devBundleDir) {
 
 function isValidCommand(name, devBundleDir) {
   if (name === "node" ||
-      name === "npm") {
+      name === "npm" ||
+      name === "npx") {
     return true;
   }