Introduce .xcode.env configuration file to source `node` (#33546)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/33546

This Diff does 2 things:
1. Removes all the remnant of the `find-node.sh` script. This allows React Native to stay agnostic from any other node manager
2. Introduces a way for the developers to specify which `node` executable they want to use, through a simple `.env` file.

## Changelog
[iOS][Changed] - This PR removes the `find-node.sh` scripts and replaces it with an `.xcode.env` file that is sourced by the script phases that needs it. The `.xcode.env` file is versioned: to customize a local environment, an unversioned `.xcode.local.env` can be used.

Reviewed By: cortinico

Differential Revision: D35317070

fbshipit-source-id: 4b400ba56aa2d574db563fa67b2008e1ddde1c59