build: explicitly mark fns to improve tree-shaking

[pi0]

Related to #129

(This PR is mainly for experimenting)

Add explicit /* @__PURE__ */ marks to improve tree-shaking with a roll-up. (it does not fully trust sideEffects: false in package.json only)

This only works if the rollup treeshake option is set to smallest and by default does not change any behavior still!

Due to ESBuild limitations for minify options, we have to disable whitespace modification which means the default size of dist/index.mjs goes from 3.96 kB to 4.86 kB.

rollup repl