feat: Build with Rolldown

[timfish]

This PR:

• Migrates away from Rollup + plugins to Rolldown
• Removes a load of files that are no longer required
• Simplifies the build configurations
• Prebuild script writes the LIB_VERSION to a file so we don't have to replace it at build time
• Changes CI so that the packages are built once and artifacts are shared to the downstream jobs
  • This is required because Rolldown won't run on the ancient Node versions we test
  • It's much faster to copy the artifacts than to run the build in every job

Bonuses

• Rollup v2 no longer in our deps
• The full CI build is 33% faster
• Even with TypeScript still doing declarations, the build step 2x faster

[github-actions]

Semver Impact of This PR

🟡 Minor (new features)

📋 Changelog Preview

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New Features ✨

• Build with Rolldown by timfish in #872

• Esbuild no longer uses unplugin by timfish in #871
• Webpack no longer uses unplugin by timfish in #870

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[JPeer264]

q: Are there any downsides to upgrade to v22 here? In the JS SDK we use v20, and I'm not sure if we can upgrade that easily (maybe we can)

[JPeer264]

Also for the record. I'm fine with a RC of rolldown being in here. If there are no issues with it ofc - but I'm not a pro in rolldown either :)