Add installation size metrics to benchmarks

[lukewhitehouse]

The benchmarks on the pnpm website are a great starting point for anyone like me who's been conducting an investigation into a dependency management strategy for my company. However, I've noticed that the benchmarks don't include installation size (in terms of the package manager's store/cache, and the node_modules folder if relevant), and I was wondering if thats something that could be considered going forward?