Migrate to pnpm

[ojeytonwilliams]

Strapi now supports pnpm and we're already running into subtle issues with the monorepo (tsc reporting errors due to type conflicts in node_modules and so on). It shouldn't be too much trouble to migrate, but strapi/strapi#15992 contains some discussion of pain points.

When migrating, make sure to remove skipLibCheck from any tsconfigs.