feat(infra): inform pnpm users “Gallium” runtime

> If you want to prevent contributors of your project from adding new
> incompatible dependencies, use `node-version` and `engine-strict` in
> a `.npmrc` file at the root of the project.

Refs: https://pnpm.io/npmrc#node-version

This way, even if someone is using Node.js v18 runtime & toolchain,
they will not be able to install a new dependency that doesn't support
our Node.js v16.13.0 (“Gallium” runtime & toolchain).

Refs: OpenINF#64

.npmrc

@@ -1 +1,3 @@
 save-exact=true
+node-version=16.13.0
+engine-strict=true