Add `compiler.define` option (#71802)
Adds a `compiler.define` option to next.config.js to statically replace
any variables during build-time for Turbopack. This is similar to
Webpack's `DefinePlugin`.
Relates to: https://github.com/vercel/next.js/discussions/71476
---------
Co-authored-by: Tim Neutkens <tim@timneutkens.nl>
