Mark package as sideEffect free in package.json

[joewestcott]

Hi @Hopding!

I'm utilising this library in a slightly different way from most, I'm just using the low-level primitives available in /core and creating PDFs semi-manually. This library is amazingly flexible, and means I don't have to re-implement difficult things like image/font loading. Hurrah!

However, despite using the ESM build (pdf-lib/es/index.js), many bundlers still bundle the entire library by default.

As a simple example, the file below only has one pdf-lib import:

// in.js
import { PDFRef } from "pdf-lib";
console.log(PDFRef);

Yet bundling with ESbuild...
esbuild --bundle ./in.js --minify --outfile=out.js
...gives an output size of 425.8kb, which is the whole library!

If pdf-lib declared itself as side-effect free, bundlers can be more aggressive with tree-shaking, and remove unused exports. I believe pdf-lib can do so, because all modules are self-contained and don't modify each other or global scope.

The same test with this PR applied... 13.9kb!

For more info, here's Webpack's documentation on this setting:
https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free

Finally - thank you for your hard work on this library! 👌