Migrated build system to tsup

[mattpocock]

Also contains work in #569, merge that first.

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[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 https://github.com/supermacro/neverthrow/pull/569 is clarified.

[jalavosus]

@supermacro It appears that #569/#618 were merged. Any chance you could circle back to this?