Add a packageManager property to root package.json to enable use of corepack

[ericallam]

This will make it easier for contributes to download the repo and install packages using the required package manager.

Bonus points for adding an .nvmrc to make it easy to use the correct Node.js version.

[msfstef]

We include a .tool-versions configuration file for the asdf runtime manager which includes the pnpm version used for the workspace.

This allows us to specify the package manager, node version, as well as elixir version all in one place.