improve `pnpm run package`

[marcoieni]

The size of the dist folder is huge. Tree-shaking would avoid copy pasting all the files imported and instead copy just the items imported.

Tree-shaking could also prevent bugs. E.g., if post.ts imports a constant from main.ts, then the code from main.ts is copy pasted in post.js, which causes bugs.

[marcoieni]

this is blocked by actions/toolkit#1436
I.e. from my understanding, using "@actions/core" forces us to use "@rollup/plugin-commonjs" and prevents rollup from doing tree-shaking.

EDIT: now we use tsup, but still it would be nice to get tree-shaking.

EDIT2: we now use tsdown