feat: support custom LinearGradient component via prop

[androxxe]

Problem

Users of Expo (or any other environment) cannot use this library out of the box
because it hardcodes react-native-linear-gradient, which is not compatible with
Expo's expo-linear-gradient.

Solution

Accept a LinearGradient component as a prop, allowing users to pass any compatible
gradient library. Defaults to react-native-linear-gradient so existing usage is
fully backwards compatible.

Usage (Expo)

import { LinearGradient } from 'expo-linear-gradient';
import SkeletonPlaceholder from 'react-native-skeleton-placeholder';
<SkeletonPlaceholder LinearGradient={LinearGradient}>
  <SkeletonPlaceholder.Item width={120} height={20} />
</SkeletonPlaceholder>
Changes
- src/skeleton-placeholder.tsx — added LinearGradient prop with react-native-linear-gradient as default
- package.json — marked react-native-linear-gradient as optional peer dependency
- README.md — documented the new prop and added Expo usage exampl