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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
If you're using javascript I can't recommend geotic highly enough. I've used it on a bunch of games myself and have always had a pretty good experience. It's not a pure ECS architecture as it encourages logic in your components and events but that's mostly because it's based on talks given by the devs from ADOM and COQ - both of which seem to use ECS-like architecture. The repo has some nice example projects worth a look.
I also used bitecs which is a far more "pure" implementation of ECS. It was fine and the developer is super active, but personally I found that geotic included some patterns out the box that were kind of a pain to reimplement on my own.