Skeleton Loader component

**Describe the solution you’d like**

Add a built-in Skeleton Loader component to react-native-paper that can be used as a placeholder while content is loading.

The component should align with Material Design principles and the existing Paper theming system (colors, dark mode, shape, animation preferences).

This would provide a consistent, first-class loading experience without requiring third-party libraries.

**Describe alternatives you’ve considered**

Currently, skeleton loading is typically implemented by:

Using third-party libraries (which may not integrate well with Paper’s theme system), or

Manually composing View components with animated opacity/backgrounds, which leads to duplicated, inconsistent implementations across apps

Neither approach feels ideal for a UI library that already provides polished, opinionated components.

**Additional context**

Many modern design systems include skeleton loaders as a standard pattern, and they are increasingly preferred over spinners for content-heavy screens. Having this as part of react-native-paper would improve developer experience and visual consistency across applications using the library.

Comme aucun fichier, test ou point d’entrée n’est indiqué dans l’issue, commencez par localiser les composants React Native Paper existants ainsi que ses modèles de theming et d’animation. Le travail sera considéré comme terminé lorsqu’un Skeleton Loader intégré disposera d’une API définie et alignée sur Material, qui s’intègre aux couleurs, au mode sombre, à la forme et aux préférences d’animation.

Rédigé par le modèle d'indexation à partir du texte de l'issue.