feat: add viewport size to LayoutContext class (#56209)

Summary:
Add viewport size to `LayoutContext` and wire it through Android and iOS layout setup so viewport dimensions are available during layout.

This was extracted from the larger `calc()` work in PR https://github.com/facebook/react-native/pull/56162.

## Changelog:

[GENERAL] [ADDED] - Add viewport size to LayoutContext

Pull Request resolved: https://github.com/facebook/react-native/pull/56209

Test Plan:
- This change is limited to internal layout plumbing in React Native core and does not alter external behavior on its own.
 - Validation for the actual `calc()` use cases will be covered in the follow-up work that consumes this plumbing.

Reviewed By: christophpurrer

Differential Revision: D98004758

Pulled By: NickGerleman

fbshipit-source-id: 3fd6257b2c280442a41308af3e6eff30b51a3397