Migrated build system to tsup

[mattpocock]

Also contains work in #569, merge that first.

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[dmmulroy]

@mattpocock could you please add some context to why tsup is a good/better solution beyond just being "modern"? Thanks!

@supermacro @m-shaka I am a fan of tsup and in favor of this change once justification and context is added to the description

[mattpocock]

The main appeal of tsup is that I trust the declaration files it generates. It also generates one per output type by default (which is recommended by Andrew Branch of the TS team) which means the exports map can be simpler.

If you're happy with your current rollup-based setup, that's fine - but it seemed a bit custom and funky so I thought a low-config, well-maintained option would make sense.

[supermacro]

refraining from diving into this PR until convo for #569 is clarified.