Add option to use pnpm as the package manager

[MikeAlexMartinez]

I like to have various workshops installed at any one time and refer back to them and play around.

Having different workshops and courses installed means each directory has it's own node_modules which uses a lot of memory! (Kent and the team keep pumping out too much great educational content! 😅 ).

Using Pnpm would enable users to only install non-duplicated versions of libraries instead. It would also speed up most installs and updates.