Add support during template creation for pnpm and bun (#2007)

This pull request add support for bun and pnpm during template creation. ``` D:\___MATH\observablehq-framework>npm run observable create > @observablehq/framework@1.13.3 observable > tsx --no-warnings=ExperimentalWarning ./src/bin/observable.ts create T observable create v1.13.3 | * Welcome to Observable Framework! 👋 This command will help you create a new app. | When prompted, you can press Enter to accept the default value. | | Want help? https://observablehq.com/framework/getting-started | o Where should we create your project? | .\hello-framework | o What should we title your app? | .\hello Framework | o Include sample files to help you get started? | Yes, include sample files | o Install dependencies? | Yes, via pnpm | o Initialize git repository? | No | o Installed! 🎉 | o Next steps… ----------+ | | | cd .\hello-framework | | pnpm approve-builds | | pnpm run dev | | | +------------------------+ | — Problems? https://github.com/observablehq/framework/discussions D:\___MATH\observablehq-framework> ```