Support for TypeScript project references

The vanilla TypeScript compiler has a feature called project references that makes it more convenient to work on composite/monorepo projects that would otherwise need multiple watch processes, and also for making sure they rebuild when a dependency changes.

It would be nice to have support for this in tsup.

Basically it might just need tsup to know how to correctly order the build of dependent project's d.ts.

https://www.typescriptlang.org/docs/handbook/project-references.html