Update and merge backend and frontend into monorepo

[1stl0ve]

This update combines the previous frontend and backend repositories into a single monorepo. It additionally includes several significant updates:

• Migrates implementations from javascript to typescript
• Changes build system to pnpm
• Adds database (postgres) for storing exercises; defines schema with prisma
• Implements a frontend interface for adding/editing/cloning exercises
• Adds initial "privileged" access for single user via password authentication
• Re-designs backend API to support exercise creation, modification, deletion, validation, and submission.