[Feature]: Migrate away from tsup

[ashquarky]

Checked Existing

• I have checked the repository for duplicate issues.

What feature do you want to see added?

Move away from tsup to another bundler. tsdown is the recommended replacement, but we can take an opportunity to review all our options (especially since we are bundling both client and server stuff in a single build, which seems to be a bit uncommon).

Why do you want to have this feature?

tsup is no longer maintained and was not a suuuper great fit in the first place.

Any other details to share? (OPTIONAL)

No response

[ashquarky]

I did attempt to use tsdown and found some issues:

• When using multiple entry points (e.g. juxt.js, debug.js; which we use to make debug.js optional), IIFE output - needed for ctr and portal since they're old browsers - doesn't work at all. It asks to disable code splitting, but then says code splitting is needed for multiple entry points. They have an open feature request for this. A workaround is to duplicate the build configs for each file.
• Limited support for CSS bundling - especially resolving url() which we use in ctr for dataurls. These are all known things on their end - Feedback on native CSS support rolldown/rolldown#4271 , CSS Support rolldown/tsdown#472 . Maybe possible to use a plugin instead but I couldn't work out how to do this.
• Need to adjust our in-repo ESBuild plugins (expected for any migration)

I think, mostly due to the lacking CSS support, tsdown/Rolldown isn't quite ready for us, which is fine - it's still in dev anyway.