chore: pin used pnpm version in package.json

[mehcode]

For documentation purposes at least, the expected pnpm version should be declared in package.json. See https://nodejs.org/api/packages.html#packagemanager

I would recommend updating to pnpm v9 when time allows. I would have done it here, but it involves an automatic reformat of the lockfile and has a high chance of conflicting any current work.