Deprecate core/scripts/js/babel-es6-build.js for removal from 10.0.x

Code looks good to me.

+++ b/core/scripts/js/babel-es6-build.js
@@ -22,6 +22,8 @@ const changeOrAdded = require('./changeOrAdded');
+console.warn('⚠️  yarn `build:js` command is deprecated in drupal:9.4.0 and is removed from drupal:10.0.0. This command is no longer needed in Drupal 10.0.0.️');

Ideally we would display this message at the end of the file processing so it's clearly visible. But the way things are done we don't have a good way of deferring the message after all the files have been processed (unless we rewrite a bunch of things to use promises). So displaying on top is fine.

There's a bit of chicken and egg with this issue and #3278415: Remove usages of the JavaScript ES6 build step, the build step itself, and associated dev dependencies but I don't think we can deprecate this in 9.4.x until it is no longer necessary in D10.

Discussed with @lauriii - we need to commit this before #3278415: Remove usages of the JavaScript ES6 build step, the build step itself, and associated dev dependencies as that one should be delayed for as long as possible. So I proposed we change the message to:

yarn `build:js` command is deprecated in drupal:9.4.0 and will be removed from drupal:10.0.0. This command is no longer needed in Drupal 10.0.0 once https://www.drupal.org/project/drupal/issues/3278415 is committed.️

Yes this is not in the standard deprecation format but it does tell the truth and explain why we are still using the command in D10 today. Once #3278415: Remove usages of the JavaScript ES6 build step, the build step itself, and associated dev dependencies lands and removes the file this message will be in 9.4.x and 9.5.x and still helpful as people will be able to see what we did to remove core's dependency on it if they are using the tool for contrib or custom development.

Committed and pushed 8620063ae0 to 10.0.x and 004fa86e19 to 9.5.x and eb3ef8c00a to 9.4.x. Thanks!