Switching from unbuild to tsdown which is basically about to become the new de facto standard for bundling libraries in the JS ecosystem. It's fast and easy to use (no config file required).

Related changes:

• Adding the minimum required Node.js version via engine.node in the package.json file. This is used by tsdown to determine the build target ¹. This way, we can easily ensure that the correct code is generated.
• Adjusting and simplifying the module/types/exports definitions in the package.json file. types should be set to the ESM definitions as the main type of the package "module". We don't need to explicitly define the types under exports as those are automatically resolved by TypeScript.
• Also renaming the tsc script to the more explicit typecheck.