🤖 docs: Reference regenerator-runtime instead of @babel/polyfill.

Replace all references to @babel/polyfill by references to regenerator-runtime/runtime.

These changes were automatically generated by a transform whose code can be found at:
  - https://github.com/aureooms/rejuvenate/blob/e086830dfe927964be93f46b8ecdacd2597eb487/src/transforms/docs:replace-scoped-babel-polyfill-with-regenerator-runtime.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).
+[regenerator-runtime/runtime](https://babeljs.io/docs/usage/polyfill).
 ```js
-require( '@babel/polyfill' ) ;
+require( 'regenerator-runtime/runtime' ) ;
 // or
-import '@babel/polyfill' ;
+import 'regenerator-runtime/runtime' ;
 ```
 
 Then