Add 'find-up' dev dependency

Summary:

• We need this in scripts/get_cargo_path.js in the child diff.
• The 5.0.0 is the last non-ESM version of this package, and our .lintstagedrc.js is a CJS file.
• yarm.lock remains unchanged as this version was already a transitive dependency.

Test Plan: yarn cleaninstall doesn't affect yarn.lock

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6314