[chore] add pnpm version for corepack compatibility

[waynevanson]

This is aimed as contributors.

When I use my version of PNPM I get a different lockfile on install.

Can we use corepack to figure out what to use in package.json?
https://nodejs.org/api/packages.html#packagemanager

+  "packageManager": "pnpm@9.1.4+sha1.2432063d815cfa88fd9fef1d85a445e3f609851d"

Here's what I get when I run pnpm i

diff --git pnpm-lock.yaml pnpm-lock.yaml
index eddeef4..aa6ea16 100644
--- pnpm-lock.yaml
+++ pnpm-lock.yaml
@@ -1,1980 +1,6349 @@
-lockfileVersion: 5.4
+lockfileVersion: '9.0'