migrate from tsup to tsdown

[theoephraim]

would like to migrate from tsup to tsdown. Mostly should be straightforward, but a few additional related things come to mind:

• currently we spit out ".js" esm files because of tsup. Would rather transition everything to .mjs and .cjs for clarity
• tsdown has better control over leaving node: prefixes. I think we have some workarounds for this that we may be able to remove

Likely more subtle things will come up as we go.