Use optionalDependencies instead of postinstall script

[jcbhmr]

This has the benefit of:

• Being more easily downloadable from an offline cache
• Conforming with ecosystem conventions (ex: optionalDependencies is what https://napi.rs uses)
• Not requiring postinstall permissions (if user disables npm scripts optionalDependencies still works)

First trial release is https://www.npmjs.com/package/typst/v/0.10.0-6

works like a charm on my Windows 10 x64 PC and in WSL Linux x64 🎉🎉🎉