transpile to ESM with tsdown

[dynst]

#1004

Blocked on

#1758 #1785 #1805 #1833

And possibly a hilarious interaction with using yarn pnp instead of node_modules/.

https://yarnpkg.com/getting-started/qa#does-yarn-support-esm https://github.com/yarnpkg/berry/discussions/4044#discussioncomment-11395531