Mitosis (https://github.com/BuilderIO/mitosis) is a fantastic project, but is still in its early stages, and in need of some build tooling. It allows you to "write once, run everywhere" in the sense that you can write a UI component and have it compiled to a React component for use in a React project, a Svelte component for use in a Svelte project, and so on. It does not, however, support bundling.create-multi-lib (run like so `npx create-multi-lib <your-lib-name>) is my attempt to contribute a much needed bundling-layer on top of Mitosis.
It also comes with E2E testing included, and your resulting packages include type-declarations.