fix(core): add React as optional peer dependency to fix monorepo resolution

[jonlambert]

Summary

Adds React and React DOM as optional peer dependencies to enable proper dependency resolution in monorepo environments with multiple React versions.

Why?

We had an issue when using the same version of swiper in a monorepo with different React versions. When rendering server-side, we'd get a hook warning with the wrong version of React resolved. pnpm wasn't linking the correct version of React because it doesn't know swiper depends on it.

In our project, I've used a bit of middleware in .pnpmfile.cjs to add react + react-dom to the peer dependencies inside swiper. This has let pnpm know that it needs to link the right version of React rather than just resolving the nearest version:

(Our project's .pnpmfile.cjs)

module.exports = {
  hooks: {
    readPackage(pkg) {
      if (pkg.name === 'swiper') {
        pkg.peerDependencies = {
          ...pkg.peerDependencies,
          react: '>=16.8.0',
          'react-dom': '>=16.8.0',
        };
        pkg.peerDependenciesMeta = {
          ...pkg.peerDependenciesMeta,
          react: { optional: true },
          'react-dom': { optional: true },
        };
      }
      return pkg;
    },
  },
};

Thought it might be useful for others so I thought I'd open this PR! 🙂