Migrate build system from tsup to tsdown

[dcramer]

Summary

• Replace tsup with tsdown as the build system for the project
• Update build configuration and scripts accordingly
• Ensure all existing functionality is maintained

Motivation

Migrate the build system from tsup to tsdown to [add specific benefits/reasons for migration here].

Tasks

• Remove tsup dependencies from package.json
• Add tsdown as a dependency
• Convert tsup.config.ts to tsdown configuration
• Update build scripts in package.json
• Test build output to ensure compatibility
• Update any documentation referencing the build system

Notes

Current tsup configuration can be found in tsup.config.ts