Add shell script to automate full Inertia.js dependency installation and package setup

[michaelnabil230]

This PR introduces a new shell script (install.sh) to streamline the development setup process for the Inertia.js monorepo.

Key Features:

• Verifies the existence of pnpm and provides installation instructions if missing.
• Installs root dependencies using pnpm install.
• Recursively installs dependencies for:
  • All packages in ./packages/*
  • All playgrounds in ./playgrounds/*
  • The main test app (tests/app)
  • Individual test apps in packages/react/test-app, packages/svelte/test-app, and packages/vue3pp
• Builds all packages using pnpm build:all.
• Displays a summary of useful pnpm commands for development and testing.

This script helps reduce setup errors, ensure consistent environments, and improve onboarding efficiency for contributors.

[pascalbaljet]

Thanks for this, but you don't need to run pnpm install in all the subdirectories. pnpm takes care of that since we're using workspaces. I've recently rewritten the Contributing Guide, and since the move to pnpm, it's much easier to get started. So I'd rather have people reading that document and fully understanding the structure than another script that we have to maintain.