Adds clean-js jake command to JakePackage and updates building instructions (#25324)

The clean-js jake command helps remove all minified js files that get generated using the minify bundle jake command

By running clean-js before running the minify command, we ensure that the tinymce files are consistent after being rebuilt/minified.

This is helpful with multiple app servers that are applying the same changes to the TinyMCE editor

This ensures that no matter on which machine the files are rebuilt, the resulting minified plugin files are consistent among all

common/static/js/vendor/tinymce/BUILD_README.txt

@@ -3,9 +3,13 @@ Instructions for creating js/tinymce.full.min.js
 1. Ensure that the dependencies (NodeJS, Jake, and other dependencies) are installed. If necessary,
    install them per the directions on https://github.com/tinymce/tinymce/tree/4.0.20.
 2. Unzip edx-platform/vendor_extra/tinymce/JakePackage.zip into this directory (so that Jakefile.js resides in this directory).
-3. Run the following command in the tinymce directory:
-   jake minify bundle[themes:modern,plugins:advlist,anchor,autolink,charmap,code,codemirror,contextmenu,image,insertdatetime,link,lists,media,paste,print,save,searchreplace,table,textcolor,visualblocks]
-4. Cleanup by deleting the Unversioned files that were created from unzipping jake_package.zip.
+3. Clean install the dependencies that were unzipped
+   npm ci
+4. Run the following command in the tinymce directory:
+   npx jake clean-js
+5. Run the following command in the tinymce directory:
+   npx jake minify bundle[themes:*,plugins:*]
+6. Cleanup by deleting the Unversioned files that were created from unzipping jake_package.zip.
 
 Instructions for updating tinymce to a newer version: