This commit gets rid of rollup-plugin-hypothetical in favor of a simpler
inline Rollup plugin that fits our need and is compatible with Windows.

Fixes jquerygh-4548
Closes jquerygh-4549

Summary

Checklist

• All authors have signed the CLA at https://cla.js.foundation/jquery/jquery
• New tests have been added to show the fix or feature works
• Grunt build and unit tests pass locally with these changes
• If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com