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

[jlguenego]

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>

[jlguenego]

By the way, it was motivated by solving #2007

[jlguenego]

Added also a documentation update.

Updated to have bun dev and pnpm dev instead of bun run dev and pnpm run dev