[Internal] Support building with `pnpm`

# Overview

Currently, building hackportal requires you to use `npm`. As alternative build tools like `pnpm` become more popular, it would be helpful to provide support for them out of the box. This will enable faster installs, faster builds, and lower disk storage sizes on developer computers. `npm` will still be used for deployment, but support for `pnpm` will be a great developer convenience due to several of `pnpm`'s features.

# Proposal

This wouldn't be a difficult change. I use `pnpm` to develop hackportal. Supporting `pnpm` out of the box mostly amounts to explicitly specifying peer dependencies that `npm` installs automatically. The reason this is an issue at all is explained in `pnpm`'s [pnpm vs npm](https://pnpm.io/pnpm-vs-npm) docs page.