Refactor IS_NATIVE and global._IS_FABRIC

[bartlomiejbloniarz]

Summary

This PR removes IS_NATIVE in favor of SHOULD_BE_USE_WEB to be more consistent with PlatformChecker. Additionally isFabric() was added to PlatformChecker.

Test plan

[tomekzaw]

Looks good to me 👏

After reading this PR, I would say that sometimes it's convenient to have IS_NATIVE because it's easier to understand than !SHOULD_BE_USE_WEB but I think it's a step in the right direction anyway.

BTW Does it mean that we no longer need global._IS_FABRIC on the UI runtime? Technically, we could remove injecting it but is it safe to do so? Also, if we leave it and add 'worklet'; directive in the isFabric function, this will also work. Are there any use cases where we need to check if Fabric is enabled on the UI thread?

[bartlomiejbloniarz]

@tomekzaw as mentioned offline, global._IS_FABRIC is currently not used on the UI thread but I think we should keep it around until at least we are done with layout animations on Fabric