Support pnpm as a JavaScript package manager option

[yusufkandemir]

Currently, post-install only offers between yarn and npm(pro only) to install JavaScript dependencies.

Add pnpm as another option, preferably to this open-source template and not the pro (😅).

To clarify, it's possible to use pnpm with spin, but with extra steps. The users can continue the script with yarn, then delete node_modules/ and yarn.lock. Afterward, they can set up and use pnpm.
I am not sure how good the solution is, but I've created Dockerfile.node, used the official node image as the base, installed pnpm inside it, and used that image in the compose file instead.

[jaydrogers]

Add pnpm as another option, preferably to this open-source template and not the pro (😅).

This would most likely be added to Pro, not open source. I wish I could give everything away for free, but that can only work in a world where all my expenses are free (obviously not a realistic scenario).

I hope all of our other contributions and vast selection of open source projects provide enough value to show our commitment to open source. Anything we earn from Pro only help us build the open source versions better.