Use tsdown for building the library

[timokoessler]

• Would allow us to bundle dependencies, e.g. unplugin, to ensure we still have 0 npm dependencies
• Reduce lib size & loading time

Todos

• [x] Some benchmarks are trying to import internal files from the build directory
• [x] Rolldown does not work on old Node.js versions

[codecov[bot]]

Codecov Report

:x: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
library/helpers/getLibraryRoot.ts	75.00%	1 Missing :warning:

:loudspeaker: Thoughts on this report? Let us know!