Clean-up npm package

[rgaudin]

Here's the output of npm publish --dry-run

npm notice
npm notice 📦  @openzim/libzim@2.2.5
npm notice === Tarball Contents ===
npm notice 1.5kB   package.json
npm notice 4.1kB   .clang-format
npm notice 21B     .env
npm notice 2.2kB   binding.gyp
npm notice 1.2kB   bundle-libzim.js
npm notice 275B    Changelog
npm notice 543B    CPPLINT.cfg
npm notice 2.0kB   download-libzim.js
npm notice 359B    jest.config.js
npm notice 35.1kB  LICENSE
npm notice 2.3kB   README.md
npm notice 281B    tsconfig.json
npm notice 853B    tslint.json
npm notice 677B    .github/FUNDING.yml
npm notice 119B    build/binding.Makefile
npm notice 23.4kB  build/gyp-mac-tool
npm notice 13.5kB  build/Makefile
npm notice 3.0kB   build/Release/.deps/Release/obj.target/zim_binding/src/article.o.d
npm notice 3.6kB   build/Release/.deps/Release/obj.target/zim_binding/src/module.o.d
npm notice 3.5kB   build/Release/.deps/Release/obj.target/zim_binding/src/reader.o.d
npm notice 3.0kB   build/Release/.deps/Release/obj.target/zim_binding/src/writer.o.d
npm notice 621B    build/Release/.deps/Release/zim_binding.node.d
npm notice 15.7MB  build/Release/libzim.6.dylib
npm notice 693.0kB build/Release/obj.target/zim_binding/src/article.o
npm notice 178.0kB build/Release/obj.target/zim_binding/src/module.o
npm notice 535.6kB build/Release/obj.target/zim_binding/src/reader.o
npm notice 539.0kB build/Release/obj.target/zim_binding/src/writer.o
npm notice 148.2kB build/Release/zim_binding.node
npm notice 5.9kB   build/zim_binding.target.mk
npm notice 167B    dist/index.d.ts
npm notice 321B    dist/index.js
npm notice 139B    dist/zim.d.ts
npm notice 547B    dist/zim.js
npm notice 791B    dist/ZimCreator.d.ts
npm notice 7.9kB   dist/ZimCreator.js
npm notice 402B    dist/ZimReader.d.ts
npm notice 5.3kB   dist/ZimReader.js
npm notice 8.1kB   src/article.cc
npm notice 3.9kB   src/article.h
npm notice 323B    src/module.cc
npm notice 3.4kB   src/reader.cc
npm notice 916B    src/reader.h
npm notice 1.9kB   src/writer.cc
npm notice 953B    src/writer.h
npm notice === Tarball Details ===
npm notice name:          @openzim/libzim
npm notice version:       2.2.5
npm notice package size:  7.2 MB
npm notice unpacked size: 18.0 MB
npm notice shasum:        67b3ea6d163119d81467f947682a6a8020ada545
npm notice integrity:     sha512-W2Ubwk0OwY1sN[...]3O+nfa5gGyBEA==
npm notice total files:   45

As you can see, it contains a lot of files that probably shouldn't be submitted: all of the build folder for instance (maybe others). This is all present in the current online package as can be verified with npm pack @openzim/libzim