fix(babel-preset): preserve Platform.select initializers

[mfkrause]

Summary:

The React Native Babel preset 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 on iOS however.

This fix preserves both side effects by testing for purity.

Changelog:

[GENERAL] [FIXED] - Preserve side effects from discarded Platform.select property initializers in the Babel preset.

Test Plan:

Added a regression test and ran existing tests, linter and formatter.