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.

Issue 中没有指定文件、测试或入口点，因此请先定位现有的 React Native Paper 组件及其主题和动画模式。完成的标准是提供一个内置的 Skeleton Loader，具有定义明确且符合 Material 的 API，并能与颜色、深色模式、形状和动画偏好集成。

由索引模型根据 Issue 内容生成。