feat: detect the package manager used to customise next steps

[JasonTame]

Resolves: #2

Adds some utility functions which detect the package manager used to run create-rwsdk and customises the next steps section accordingly:

Pnpm

Next steps:
  cd my-project
  pnpm install
  pnpm dev

Yarn

Next steps:
  cd my-project
  yarn install
  yarn dev

Npm (default)

Next steps:
  cd my-project
  npm install
  npm run dev