Preserve Platform.select initializer side effects - #1903

Summary

Metro replaces Platform.select({...}) with the selected property during production transforms. JavaScript evaluates every object property initializer before calling Platform.select, so this can silently remove side effects from non-selected properties.

For example:

Platform.select({
 ios: selected(),
 android: discarded(),
});

JavaScript would run both side effects, selected() and discarded(). The plugin's current behavior removes discarded however.

This fix preserves both side effects by testing for purity.

Changelog:

 - **[Fix]**: Preserve side effects from discarded `Platform.select` property initializers

Test plan

Added a regression test, and ensured existing tests, linter and formatter pass.