Upgrade to Vite 6

Chains the earlier majors: a project several majors behind needs only this recipe, which also runs the code changes of the earlier majors in order, and upgrades the dependencies once, straight to 6.x, so no intermediate version has to resolve. Upgrade vite to 6.x and the official plugins that must match it in package.json: @vitejs/plugin-react 4.x, @vitejs/plugin-react-swc 3.x, @vitejs/plugin-vue 5.x, @vitejs/plugin-legacy 6.x and @sveltejs/vite-plugin-svelte 5.x. In vite.config and vitest.config files it marks the Vite 6 changes that are not mechanical, such as resolve.conditions and json.stringify (FindVite6Changes). Not done by this recipe: raising engines.node, the changes that are only marked, and community plugins.