Use pnpm as default package manager

[robinheinze]

After the recent Sha1-halud attacks, we're moving away from yarn 1 and npm and generally recommend pnpm.

I think we should:

• update documentation and README's to use pnpm in example commands
• remove npm as an option in the CLI package manager prompt
• specify in CLI package manager prompt that pnpm is recommended
• specify in CLI package manager prompt that yarn option is Yarn 4.

[coolsoftwaretyler]

I'm in support of this - we've been using pnpm on my current client project and I enjoy the experience day to day, along with its security benefits.