Rename package to release under the `@stellar/` npm scope.

[Shaptic]

This change makes several updates:

• the package was renamed to @stellar/js-xdr
• the publish script was modified to continue publishing to js-xdr for an easier transition
• a deprecation message was added to the old location
• yarn upgrade-interactive --latest was executed to ensure all dependencies are at their latest versions
• babel.config.js was renamed to babel.config.json for consistency with the other libraries
• the version was bumped to 3.0.1 to trigger the publish action