Experimental DTS and Multi-entrypoint builds

I really like the option to use @microsoft/api-extractor for DTS types. However, it doesn't work for all cases. It seems that it's currently only built for single entrypoint builds, see this section here:

https://api-extractor.com/pages/setup/configure_rollup/#an-important-limitation

And another thing, it doesn't seem to be copying the emitted dts files into the dist directory, even though it seems that it should be.

I also wanted to comment, Tsup is an incredible library and it's made building esm/cjs libs a breeze. Thank you very much for your work 🙏🏻

Start by reading the linked API Extractor limitation and reproduce a multi-entrypoint TypeScript build with DTS enabled. Trace where emitted declaration files are handled and whether they reach dist. Done means DTS builds support multiple entrypoints and the emitted declaration files are copied into dist.

Written by the indexing model from the issue text.