Meta: Build system overhaul

[CommandMC]

Bringing a lot of things back up to the latest and greatest in JS land

Performance differences (measured on a rather old Thinkpad T430, so your times will probably be faster):

• Package install time ((yarn,pnpm) install): 17.46s -> 2.4s
• Build time (build script): 6.9s -> 11ms
• Test time: (test script): 6.34s -> 1.56s

[CommandMC]

• .pre-commit-config.yml is updated: https://github.com/CommandMC/serenity-discord-bot/blob/9c881c3897a22d9ac45225d1f91b075b25e47c64/.pre-commit-config.yaml#L30.
• npm is generally fine to use, as long as you don't use it to manage packages (e.g. running scripts like start or lint is fine). In this case, I don't think Heroku preinstalls pnpm, so I just left that file as-is.
• Yes, no changes are necessary there (GH Docs on supported package managers). I'm actually not sure whether the "Supported versions" list there is accurate, I've been using pnpm 9 on another project and am still getting dependabot updates.