feat(babel): add process.env.EXPO_OS

[EvanBacon]

Why

Add support for detecting the platform without needing to import react-native or react-native-web.

How

Add a babel plugin to replace process.env.EXPO_OS with the current caller.platform environment variable. Unlike Platform.OS which performs fake minifying ahead of time, this value cannot be used to remove unused require statements. We can fix this when we add tree shaking in the future.

This transform can be used in any node module and is not limited to application code, similar to NODE_ENV.

Test Plan

• Added tests for the babel transforms and sanity tests for minification to better understand the limitations of code removal in Metro.

[ide]

Not going to block this PR but I think it would be better to come up with an alternative to using process.env (or process for that matter). This has nothing to do with the app process nor its environment. Something like "use os=ios" would perhaps be better, or import.meta.expoBuildConstants.TARGET_OS. Again, not to hold this up but using process.env for not-the-app-process's environment to me is an example where more clarity of mind was needed in JS conventions many years ago.

[EvanBacon]

import.meta.env.EXPO_OS would be better, but we don't have any modules story atm.