fix: define packageManager field

[yanthomasdev]

Summary

Basically, this adds the packageManager field to avoid dependencies being installed using yarn or pnpm. The way they hoist and symlink packages all have differences, and if you installed the project using something else than npm (in my case, pnpm) it would lead to an issue when trying to run npm run build.

This is to improve the developer experience especially for first-time contributors, since many expect every repository to work fine nowadays with pnpm, for example. In a follow-up PR, we can definitely change to pnpm as default since it has better monorepo support than npm.

Test plan

If you try to run yarn install or pnpm install it will be blocked through corepack, e.g.

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