🤖 docs: Reference scoped babel packages.

Replace all references to unscoped babel-* packages by references to scoped @babel/* packages.

These changes were automatically generated by a transform whose code can be found at:
  - https://github.com/make-github-pseudonymous-again/rejuvenate/blob/a44c42967442d65fd14272682fb84a6f32feb841/src/transforms/docs:replace-unscoped-babel-packages-with-scoped-babel-packages.js
Please contact the author of the transform if you believe there was an error.

Author: a-flying-potato

doc/manual/usage.md

@@ -1,11 +1,11 @@
 # Usage
 
 The code needs a ES2015+ polyfill to work, for example
-[babel-polyfill](https://babeljs.io/docs/usage/polyfill).
+[@babel/polyfill](https://babeljs.io/docs/usage/polyfill).
 ```js
-require( 'babel-polyfill' ) ;
+require( '@babel/polyfill' ) ;
 // or
-import 'babel-polyfill' ;
+import '@babel/polyfill' ;
 ```
 
 Then